The length-to-weight ratio (also know as the "Drop Weight") is the difference between the bat's length and the bat's weight.
Bats are generally measured in inches and ounces. So for an example, if a bat is 30 inches long and weighs 20 ounces, it will be referred to as a -10 or a "drop 10".
In high school and college baseball all bats must be -3's. But in youth baseball and in fastpitch softball, the bat's length-to-weight ratio is an important attribute when selecting a bat. A bat with a greater length-to-weight ratio will have a little swing weight, which enables the player to have a faster swing speed.