1) To be cock blocked is when someone prevents you of hanging or talking to a girl of interest.
Cock block can happen when you are already talking to a girl(or guy) of interest alone and someone comes up and starts to talk and hang out with you.
2) Cock block can also happen litterly, when someone prevents you from having sexual intercourse with a female of interest.
1) Moose and Kayleigh were hanging out together and Damion cock blocked Matt.
2) Matt and Kaitlyn were about to have sexual intercourse when some drunk walked into the room and cock blocked Matt.