Lerner Pub is a senior "tradition" that, because of the nature of its name (held in Lerner Hall, completed in 1999,) cannot really be that venerable). Every few weeks during senior year, you are supposed to be able to go into the dimly lit confines of the Lerner Party Space and drink either a couple cans of Heineken or Coors - before your limit has been reached, or before they quickly run out of booze (depending on the policy of the week/year).
The whole thing feels forced and lame, not the least because it consists mostly of frat boys and falling-down pre-gamed slut girls, but because it's several stories underground in a dark, cramped space, and you can get better beer at Morton Williams (which you will, when they run out). Still, it's free beer. Cheers.