Sometimes a little something extra.
Used many different ways to imply a variety of things including drugs, sex or sexual acts, money, etc.
Can be used to describe almost anything you want a little something of.
She was so wasted that she drunk dialed her ex last night for a lil sum-sum.
I told him to throw a little extra sum-sum into the sack I bought since I am such a faithful customer.
The Boy: Yo dog, whats good?
The Man: Nothin. What you doin?
The Boy: Not shit.
The Man: You wanna go behind tha dumpster n smoka lil bitter dat sum sum?
Tina: Do you smell skunk?"
Ed: Skunk? You talkin as in the animal? Or you talkin bout you smell sum sum?