A prime lens operates with only one focal length.
It is described as a fixed-focal-length lens while, in comparison, a zoom lens has varying focal lengths.
When using a prime lens there is no ability to zoom the lens to change the composition.
Therefore, if you want to change how much of your subject fits “within the frame” you may need to move closer or farther away.
In comparison, by rotating a lens zoom ring you have the ability to move your subject closer to you, or farther away.
Examples of prime lenses which lens manufacturers produce or have produced: 20 mm, 24 mm, 28 mm, 35 mm, 40 mm, 50 mm, 85 mm, 105 mm, 135 mm, 200 mm, 300 mm, 400 mm, and 600 mm.