The number (often called "drop weight") is the difference between bat length and bat weight. Take this number and add it (if the number is positive) or subtract it (if the number is negative) to the length you selected to determine the bat's weight in ounces.
Ex. a 34" bat with a -2 Weight means the bat weighs 32 ounces (34 -2 = 32)
Ex. a 34" bat with a +1 Weight means the bat weighs 35 ounces (34 +1 = 35)
Bats with a 0 Weight will be equally as heavy as long.
Not sure what weight is best for you? We strongly recommend selecting the default "Best choice for most players". This is the weight we targeted when we designed the bat and is appropriate for the vast majority of players.
Why would a player choose a heavier bat? Because the player prefers it due to his size or due to use as a training bat. Some of our pros will start out batting practice with a bat that is an ounce heavier than what they swing in a game. Additionally, the heavier the bat, the greater the durability. But for the large majority of players, the trade-off of additional durability is not worth the loss in bat speed.