A cock suck in its most basic form is for a woman (or in some undesirable incidents, a man) to take a penis (cock) into their mouths and suck it. There are many variables on this technique (see "walrus" and more).
"This one bitch was really drunk and I carried her about all night. I was getting a bit sick of her and everyone fucked off leaving just the two of us. I laid her down on her knees and said 'You owe me this'. Some might say that was a form of rape, but hell, what a fuckin cock suck"