A very sexy and sensual song with catchy soothing rhythm to relax and enjoy. Did I say sensual? I meant torrid.

Hey baby girl, I’ve been watching you all day
Man, that thing you got behind you is amazing
You make me want to take you out and let it rain
I know you got a man but this is what you should say:

Why don’t you tell him that "I’m leavin’ never to come back again"
You found somebody who does it better than he can
no more making you cry, no more them gray skies
girl we flying on the G5 G5.
I’m leavin’ never to come back again...
So go on shawty and tell him you found a new man
The one who’s so so fly
The one that keep you high
have me singing all night night night
Oh oh oh (repeat)
man, she gonna sing
Oh oh oh oh (repeat)

Now, if I talk it, girl, you know that I will walk it out
Man, I’ll put my money (money) where my mouth is
Cause you the baddest little thing that I’ve ever seen
So I’ma ask you one time, if you got a man

Why don’t you tell him that "I’m leavin’ never to come back again"
You found somebody who does it better than he can
no more making you cry, no more them gray skies
girl, we flying on a G5 G5
"I’m leavin’ never lookin’ back again... "
So go on, shawty, and tell him you found a new man
The one who’s so so fly
The one that keep you high
have me singing all night night night
Oh oh oh (repeat)
man, she gonna sing
Oh oh oh oh (repeat)

Don’t stress, don’t stress, don’t stress
Just tell him to the left left left
Don’t stress, don’t stress, don’t stress
Cause we gone & we gone & we gone
No stress, no stress, no stress
Girl you deserve nothing but the best
No stress, no stress, no stress
Girl, you need to tell him...

That "I’m leavin’ never to come back again"
You found somebody who does it better than he can
no more making you cry, no more them gray skies
girl we flying on a G5 G5
"I’m leavin’ never to come back again"...
So go on shawty and tell him you found a new man
The one who’s so so fly
The one that keep you high
have me singing all night night night
Oh oh oh (repeat)
man, she gonna sing
Oh oh oh oh (repeat)
have me singing all night
Oh oh oh oh (repeat)
man, she gonna sing (repeat)

MAN, THAT THING YOU GOT BEHIND YOU IS AMAZING= MAN is just a colloquial way of addressing a boy or a man (other possibilities are GUY in AmE and MATE in BrE, but MAN sounds more fashionable). It can be used when talking to girls too, e.g. "hi man, how you doin’"(= hello, how are you?). It can also be used as an expression to signal you're going to say something. In this case you can use it either with boys or girls (e.g. "Man, this is great"").
THAT THING YOU GOT BEHIND= he means the lowest part of her back, the place where you sit on, your bottom.
AMAZING= superb, fantastic, great, incredible (he obviously likes it)

LET IT RAIN= let it rain is a rap expression for throwing money in the air and having it rain down, so it’s a metaphor for spending a lot of money, that is, having a great and luxurious time, "living la vida loca".

I KNOW YOU GOT A MAN= I know you’ve got a boyfriend.

REPORTED SPEECH
----------------------------
He’s got a few sentences here grammatically incorrect. Well, many of the sentences in this song are grammatically incorrect, but the way he uses reported speech (when you say what somebody said) is very incorrect. Nevertheless, this way of speaking is very common in conversational English, because you are not too worried about grammar correction, but about meaning.
He says, "this is what you should say", and now we would expect direct speech (the actual words spoken), for instance
- this is what you should say: "I’m leaving"
But instead, we find another sentence:
- this is what you should say. Why don’t you tell him that...
The suggestion "why don’t you tell him that..." should be followed by reported speech. For example:
- why don’t you tell him that you are leaving?
But now we find direct speech instead:
- why don’t you tell him that... "I’m leaving"
A correct possibility would be:
- why don’t you tell him: "I’m leaving"
CORRECT ALTERNATIVES:
- this is what you should say: "I’m leaving, I found somebody who..."
- why don’t you tell him: "I’m leaving, I found somebody who..."
- why don’t you tell him that you’re leaving and that you found somebody who...

NO MORE THEM GRAY SKIES= THEM is a colloquial (or dialectal) way of saying THOSE. GRAY is an alternative spelling for GREY (the colour). THOSE GREY SKIES is a metaphor for bad times, an unpleasant situation.

WE FLYING ON A G5= "We are flying on a G5". A G5 is the Gulfstream G500 business jet, a luxury private plane, so it means that we are having a wonderful great time together, a bit like the expression "let it rain" (all very materialistic and monetary, by the way). Notice that when we’re talking about public means of transport (boat, ship, bus, train, plane, etc) we use the preposition ON (not "in").

SHAWTY= a sexy girl. A term originated in Atlanta but now spreading. Especially used as a way of addressing that girl (the same as we saw MAN before, e.g. "hi shawty, how you doin’?")

FLY= cool, sexy (e.g. "that girl is so fly!")

THE ONE THAT KEEP YOU HIGH= it should be KEEPS, because it’s 3rd person singular. To be high is to feel great, fantastic. To keep you high is to make you feel great and continue feeling great.

HAVE ME SINGING ALL NIGHT "OH OH OH"= You have me singing all night because I’m so happy... or it might be a sex metaphor

MAN, SHE GONNA SING= Man, she’s going to sing. MAN is the same as we saw before. SING OH OH OH is the same as we saw before.

IF I TALK IT OUT I'LL WALK IT OUT= If I say it, I'll do it (I mean what I say)

I’LL PUT MY MONEY WHERE MY MOUTH IS= I’ll do as I say, I’ll back up my words with actions (same as the previous sentence)

BADDEST= In standard English, BAD is the opposite of GOOD. In colloquial AmE (slang), BAD may be the opposite of BAD, so it can mean GOOD. In this case, the comparative and superlative is "badder, baddest", and not usually "worse, worst". So, as Michael Jackson said in his song, "I’m bad", means "I’m good, I’m cool". BADDEST means "great, the best".

LITTLE THING= "little" here is not describing size, but showing affection (a very common use of "little")

I’MA ASK YOU= I’m going to ask you. This form of the future is still very colloquial, but getting more and more common, especially in songs. (AmE)

DON’T STRESS= don’t get stressed

TELL HIM "TO THE LEFT"= tell him to get out of your way, to get our of your life. Put him out of your head.

CAUSE= because

WE GONE= we are gone, we left

The meaning of this song is pretty simple: boy likes girl, boy gets girl but... she’s got a boyfriend, so the boy wants the girl to leave her current boyfriend and be with him because things will be better with him.