British band One Thing performing One Direction in the streets of London.

I've tried playing it cool
But when I'm looking at you
I can’t ever be brave
'Cause you make my heart race

Shot me out of the sky
You're my kryptonite
You keep making me weak
Yeah, frozen and can't breathe

Something’s gotta give now
'Cause I’m dying just to make you see
That I need you here with me now
'Cause you've got that one thing

So get out, get out, get out of my head
And fall into my arms instead
I don't, I don't, don't know what it is
But I need that one thing
And you've got that one thing

Now I'm climbing the walls
But you don't notice at all
That I'm going out of my mind
All day and all night

Something’s gotta give now
'Cause I'm dying just to know your name
And I need you here with me now
'Cause you've got that one thing

So get out, get out, get out of my head
And fall into my arms instead
I don't, I don't, don't know what it is
But I need that one thing

So get out, get out, get out of my mind
And come on, come into my life
I don't, I don't, don't know what it is
But I need that one thing
And you've got that one thing

You've got that one thing

Get out, get out, get out of my head
And fall into my arms instead

So get out, get out, get out of my head
And fall into my arms instead
I don't, I don't, don't know what it is
But I need that one thing

So get out, get out, get out of my mind
And come on, come into my life
I don't, I don't, don't know what it is
But I need that one thing
Yeah, you've got that one thing

I'VE TRIED PLAYING IT COOL= I have tried to look brave and confident by behaving like a cool guy (but I couldn't).
TRY + to= you try to do something (but probably you can't)
  - I tried to run but my feet were too sore and I couldn't.
TRY + ing= you do something to see if you can get a particular result
  - I had to be there by ten. I tried running but it was too far and I arrived late.

RACE (heart)= Go very fast (because you feel an intense emotion: love, nerves, fright...)

SHOT ME OUT OF THE SKY= [Cupid] shot his arrow at me (= it was love at first sight)

KRYPTONITE= The alien rock that made Superman lose all his superpowers.

FORZEN= Paralyzed, unable to move.

SOMETHING'S GOTTA GIVE= This expression means that there's a lot of tension and it's impossible to continue like this, so something must definitely happen and the situation will change (for better or worse).

I'M DYING TO...= I really really want to...

THAT ONE THING= That special thing I need.

I'M CLIMBING THE WALLS= I feel very very nervous, impatient, frustrated...

I'M GOING OUT OF MY MIND= I'm getting crazy.

This is simply the account of a teenage boy who sees a girl and he instantly falls in love with her. He doesn't even know her name, but he can't live without her. He's trying to show off to impress her, to conquer her, but when he's near her he goes too weak at the knees (his emotions paralyze him) and can't do or say anything.

To cut a long story short, they are describing the perfect crush.