This is the negative of "have" in colloquial American English. Learn this song and sing along with the video.

We ain't got a cake for the birthday party = We haven't got a cake for the birthday party.

... sob, sob, sob
Sob, sob, sob, sob, sob

What's the matter, Jordan?
What's the matter, July?

We ain't got a cake for the birthday party
Why ain't you got a cake for the birthday party?
'Cause we ain't got a cake for the birthday party

Sob, sob, sob, sob, sob
Sob, sob, sob, sob, sob

What's the matter, Jordan?
What's the matter, July?
What's the matter, Elly?
What's the matter, Jack?

... etc.

I've got a cake for the birthday party
He's got a cake for the birthday party