An attachment, placed in front of any lens, that allows the photographer to shoot from a closer vantage point, relative to the subject, than the lens, alone, would allow.
The primary reason to use a close-up lens is to increase the size of the subject within the frame.
Even macro lenses and, to a greater extent, cameras that have a macro mode or function, can benefit from the addition of a close-up lens.
While close-up lenses (also called close-up adapters) can be great tools in your kit, make sure you really need them. Know the minimum focus distance of all your lenses. It may be, for the kind of photography you do, that you already have exactly what you need.
None at this time.