A prime lens has a fixed focal length, which is why it is also commonly referred to as a “fixed lens”. What this means is that such a lens has a set angle of view which can not be changed – unless you move, you can not make the image appear larger or smaller within the frame. The only way of enlarging your subject and making it fill more of the frame is by physically getting closer to it. In turn, the only way to fit more into the frame is to step back.
Prime lenses have a single specified focal length, like 50mm. They come in all kinds sizes and focal lengths, from fisheye to super telephoto. Examples of prime lenses are: Nikon 24 f/1.4G and Canon 24mm f/1.4L.