What is it?
A cup is a bowl-like indentation at top of bat which allows for a final adjustment, if needed, to achieve a proper weight on the bat. The cupping is designed to take off some weight off the bat without compromising the bat's structural integrity.
Benefits
There are several benefits to having your bat cupped. For starters the cupping allows for the bat to stand on its own if you need.
The first real benefit comes from allowing the bat to be slightly more balanced since some weight is being taken off the end.
The true benefit comes from the ability for us to use a heavier billet to start the bat making process. A heavier billet directly correlates to the density, strength, durability, and pop of the bat.
Some players think that cupping a wood bat makes the weaker or that the bat will break easier if it is cupped. Contrarily, a cupped wood bat is made from higher density wood, thus should be stronger and more durable, thus making it last longer. An uncupped bat requires a lighter billet to start off inherently less dense than if the bat were to be made with a cup.