Fluorescent filters are used to correct the greenish tone that can show up when you are shooting daylight film under fluorescent lighting. There are various types of fluorescent filters. They can balance daylight film under warm white or white type fluorescent lights and correct the color balance when using tungsten film under fluorescent lighting. Fluorescent filters are basically general purpose filters that are designed for moderate correction. They may not always provide perfect color rendition by themselves. Because fluorescent lights shine at various temperatures they often need more filtration for proper results.
To get the best results when taking photos under fluorescent lighting it's a good idea to use a color temperature meter along with color correction filters. This is the only proven method of getting exact color correction. The filters usually improve the color cast, but won't correct it entirely.
Fluorescent filters range in price from about $12 to several hundred. They come in various sizes.