Most SLR cameras, some digital and a few compacts have a threaded ring at the front end of the lens. This is used to attach coloured and special effect filters as well as other accessories to alter the light that reaches the film. The diameter of the lens' filter thread is measured in millimetres. Stepping rings can be bought to allow ones size filter to fit on a different size thread. It's better to step up, ie use a filter with a larger thread than the lens, to avoid corners being cut off resulting in dark edges to your pictures.