A booster is something that increases a positive or desirable quality.

A booster is an extra engine in a machine such as a space rocket, which provides an extra amount of power at certain times.

A booster is a small injection of a drug that you have some time after a larger injection, in order to make sure that the first injection will remain effective.

A booster is someone who supports a sports team, organization, person, or place very enthusiastically.