This basketball drill is designed to help you work on your vertical jump, as well as being able to jump quickly multiple times. In a game you might jump to contest a shot in the paint, and then immediately have to jump again to get up and rebound the basketball. If you have to load and gather yourself before being able to jump again, then you are most likely not going to be able to grab the rebound.
Along with working on your vertical, this drill also works on your hands and being able to dunk. If you are not able to dunk than you can just lay it up, and it is still working on developing soft hands. Challenge yourself each time to jump as high as you absolutely can and you will be surprised at how much your vertical, and quick jumping abilities improve.