The word ‘prime’ is attributed to any lens with a fixed focal length. In other words, it has no zoom functionality. Because they can’t zoom, it’s harder to recompose in the viewfinder – you’ll need to move closer or farther away from your subject to make it appear bigger or smaller.
But, as they’re designed to work at a specific focal length, such as 300mm, they generally have the best image quality. If you’re looking for astounding image quality then a prime lens is for you.