A booster can be pictured as whatever gets added so that something already moving goes further. The thing is not broken and not new. It is running, and something is put in beside it to give one more push: a second dose beside a fading vaccine, an extra engine beside a heavy rocket, an extra cushion beside a short child, an extra voice beside a struggling team. Because it is an addition rather than a replacement, a booster usually has a limited job and often a limited life. Once its push has been given, it drops away, wears off, or gets used up.