You can check the FAQ off the home page for the official answer but here goes.

BFA - Batters Faced Allowed - This is how many hitters a particular pitcher can face in a game before becoming tired. Rec (recovery) is how many batters a pitcher recovers on days when he does not pitch.

USE - How much of a players "allowed" at bats or innings pitched have been used. This is, IIRC, 110% of actual at bats, and 115% of actual inning pitched. Check the FAQ for the offical numbers.

REST - Simply how many games since a player last appeared in a game.

Hope this helps.
~ Elminster