UV Filter: This filter primarily serves to block ultraviolet light from entering the camera lens. While modern digital cameras are less susceptible to UV light interference, a UV filter can still be useful for protecting the lens from scratches, dust, and moisture. It also helps to improve clarity and reduce haze in certain outdoor conditions.
CPL (Circular Polarizer) Filter: CPL filters are designed to reduce glare and reflections from non-metallic surfaces like water and glass. They are often used in landscape photography to enhance contrast and saturation, particularly in skies and foliage. By rotating the filter, you can adjust the angle of polarization to achieve the desired effect.
FLD (Fluorescent) Filter: FLD filters are primarily used in fluorescent lighting conditions to correct the greenish cast that can appear in photographs taken under such lighting. They help to achieve more accurate colors by neutralizing the fluorescent light's color temperature.
The numbers you provided, +1, +2, and +4, refer to the strength of the diopter filters, with higher numbers indicating greater magnification.