How you ad-lib? What's a punch? I ain't a boxer

from Slaughterhouse (Ft. Eminem & Skylar) – Our House Lyrics 1 on Rap Genius

Ad-libs are recorded over the vocal track to add to the vocals. Sometimes they convey more information or act as a punchline (Example Eminem Get You Mad) or simply as a signature (Example Lil Jon — Whaat? Okaay! Yeaah!)
A punch can refer to technique of punching in while recording (recording mid-verse to aid breath control, pronunciation, delivery, etc.)
Also could refer to punchlines which all four members of Slaughterhouse are well known for.

To help improve the meaning of these lyrics, visit "Our House" by Slaughterhouse (Ft. Eminem & Skylar) Lyrics 1 and leave a comment on the lyrics box