A FLD filter is a fluorescent light filter designed to improve photos taken while under fluorescent lighting. Fluorescent bulbs output with spikes at several different wavelengths, and some wavelengths are not present at all.
Consequently, using fluorescent light filters can be somewhat difficult. This filter is usually better on film photography than digital photography. With digital cameras, it is easy to create various settings or just use a custom white balance. If shooting with a film camera, these are some things to look for in filters: multi-coated, high-quality glass or resin, and the filter typically comes with a protective case.