WE BACK= (coll.) We're back, we've returned.
In colloquial English the verb "to be" is sometimes dropped.

GET THAT BODIES ROCKING= Start moving your bodies, start dancing. ROCK means to move from side to side.
"That bodies"?!*+/?... yea, it's totally wrong, it should be "those bodies". The first famous rap song was called "Rappers Delight", and I think most rappers delight in breaking grammar rules just for fun sometimes. You can watch this video, which is a parody of how rappers break grammar: Bad Grammar.

THANK GOD= Be thankful to God. (if you are not a believer you can still use this phrase to express that you feel happy about the situation you're going to describe next):
- Thank God the bus arrived on time!

THE WEEK IS DONE= The week has finished = The weekend has arrived.
To be done = to finish (e.g. are you done? = have you finished?)

A ZOMBIE= (lit. a living dead) A person who moves and does things but looks like he has no emotion, no spirit and no will, just like a slow machine. (during weekdays Usher feels like a zombie, but at the weekend he comes back to life and feels alive again)

GONE BACK TO LIFE= If you go back to life, you feel alive again.

NO CONTROL OF MY BODY= If you have no control of your body you do things you don't want to do, like pee in your pants or even worse. But don't worry, Usher doesn't mean that here, he means that the music possesses him and his body starts moving alone, without thinking about it.

AIN'T= (coll. AmE) haven't, hasn't, aren't, am not, isn't
Notice that there are some ungrammatical sentences here. This non standard grammar is common in the black English used by many black rappers and pop singers. Here are some examples of non-standard grammar you can find in these lyrics:
- we back in the club = we are back...
- get that bodies rockin = Get those bodies rocking
- She don't wrestle = she doesn't wrestle
- swear I seen you before = I swear I have seen you before

CAUSE= (coll.) Because (also spelled COZ in BrE and CUZ in AmE).

DJ= Disk-jockey, the person in a disco playing the music.

DANCE LIKE IT'S THE LAST...= In colloquial English (esp. in AmE) we can use LIKE as a conjunction + present where in standard English we would use AS IF + past:
- Dance like it's the last night of your life = Dance as if it were the last night of your life = Dance like crazy = Dance a lot.

GON'= (colloquial American English) I'm going to, gonna.

GON' GET YOU RIGHT= I'm going to get you [feeling] right = You're going to feel good.

KEEP DOWNING DRINKS= (coll.) Continue drinking fast (drinking a lot).

LIKE THERE'S NO TOMORROW= As if this is your last day.
If you do something like there's no tomorrow, you do it intensely.

THERE'S JUST RIGHT NOW= Only the present moment exists, the past and the future are not real, so live intensely and fully (often used simply to say that you should have a lot of fun, which is almost the opposite idea to the original philosophy).

GON' SET THE ROOF ON FIRE= We're going to have a wild time.
If you set something on fire, you burn it (or at least you start the fire).
If you are talking about having lots of fun inside a building, we say "we're gonna set the roof on fire (or this place on fire)". If you are talking about having a great time going to different places, we say "we're gonna burn/rock this city".

GONNA= (coll.) Going to.

MOTHERF---= Many people consider this the strongest bad word in the English language, that's why they cut the end of it.

DOWN, DOWN...= This "down" is part of the phrasal verb BURN --- DOWN, which means to completely destroy with fire.

DROPS= If something drops, it suddenly goes down.

SWEAR= I swear, I'm really telling the truth (used to emphasize what we're going to say).

HEAR NO EVIL, SPEAK NO EVIL, SEE NO EVIL= A 2,500 year old Chinese code of conduct from Confucius remembering you how to behave correctly. This is traditionally represented with three monkeys, one is covering his ears with his hands, the second covering his mouth and the third covering his eyes. These are called "the no-evil monkeys" or "the wise monkeys" (see picture)

GET IT, BABY?= Do you understand it?

CHEE-HO!= (esp. in Hawaii) An expression of excitement.

THAT'S HOW WE ROLL= This is our philosophy, that's how we see life.

TIVO= (AmE slang) to record. (since it's a brand name, it is usually spelled according to the brand spelling TiVo, using the same capital letters as TV=television).
This verb comes from a famous brand of a popular TV recorder. He means that some people (he) live an exciting life and others (you) simply watch them.

MAMI= (coll. AmE, originally a Spanish word meaning mummy) Baby; shorty.

SWITCHING= Changing. But here, the rapper probably refers to the way cats or dogs wag their tails when they are happy.

DREADLOCK= One of many long thin braids of hair radiating from the scalp; popularized by Rastafarians (see picture)

WRESTLE= /resəl/ Fight using just your whole body (see picture). The sport is called "wrestling".

HEADLOCK= A wrestling hold in which the head of one wrestler is encircled and locked by the arm and body of the other.

YABADABADOO= A cry of joy similar to YAY! but more emphatic. It was made popular by the TV cartoon characters in The Flintstones (see video)

BEDROCK= The solid rock that is inside the ground under the sand or soil. But here he reminds us of the fictional prehistory city made entirely of rock, where the Flintstones lived. The real meaning of this word in this sentence is actually "make her bed rock", and TO ROCK means to move from side to side.

MAMI ON FIRE= Baby is on fire (burning with love or passion).

RED HOT= Very very hot, like a piece of iron when it's heated a lot. If a girl is hot, she is really attractive.

BADA-BING, BADA-BOOM= This expression can be used to say that you can do something very easily, in a moment. It can also mean the same as the French expression "voilá" when you finish something, if you did it quickly and easily.

MR. WORLDWIDE= I don't understand this but I know Pitbull (this rapper singer) has an accompanying tour called "Mr. Worldwide's Carnaval".

STEP IN= Enter (walking).

HUSTLER= /hʌslə*/ This word usually means "a street prostitute", but here it may either mean "a very energetic person (too energetic)" or "a person who doesn't mind doing illicit things to get what he wants". In any case, it's not a good thing.

BUT THAT YOU KNEW= But you already knew that before coming with me (so don't complain now).

DALE!= (American Spanish) /dale/ Let's go! Come on!