When shopping for a new camera lens, you have the choice of a zoom lens, which has a variable focal length, or a prime lens, which has a fixed focal length.
That means with a zoom lens that you have a range of focal lengths at your disposal, say, 24-70mm, by just turning the focus ring. With a prime lens, though, the lens stays at the same focal length (i.e., 35mm, 50mm, 85mm and so on) all the time.
And while a zoom lens might seem like the better option because of the increased focal range, there are plenty of reasons why a prime lens might be a better choice.
Prime Lenses Let in More Light