What is a carb?

Hi. Im new here (I plan to stay for a long time) and Im also new to marijuana, my friend/dealer is helping me. When I smoke with him he uses a popcan. He makes a hole in the side which he calls a carb. I know other types of smoking utensils have them. What is it?

when you are almost done your hit remove your finger from the carb. This way when the next person gets their rip they wont choke on your stale leftovers. Get it now?

Oh! Ok. Cause my buddy was just saying that it was like, you take a hit with your finger on it, then take it off and take another one which is even better then the last. Which didnt make sense cause if I take two hits he gets mad cause he thinks im being greedy.