What does AB and BF stand for in Baseball box score for pitching?

Answer 1

Usually AB is an offensive stat which stands for At Bat, meaning who is batting at the current moment.

BF usually stands for Batters Faced, used as a pitching stat next to how many batters that pitcher struck out (Batters faced is not used in computing a pitcher's ERA, or Earned Run Average. See http://en.wikipedia.org/wiki/Earned_run_average)

Answer 2

AB stands for "at bats" which is how many times a batter gets to go up to the plate and hit the Baseball. Bf is "batters faced", which is the number of batters pitched to (pitching stats)