A camera lens filter can be clear, gray, graduated, or a variety of different colors. They’re used to alter the light that travels through the lens to the image sensor, and they’re used for a variety of effects.
Initially, photography filters were used in film photography to enhance black and white photography. By adding a different colored filter, the photographer could add depth, improve contrast, and reduce light glare that could ruin an image.
Modern lens filters still work with DSLR cameras, but many DSLR cameras have features built into the shooting modes, camera settings, and even in the actual hardware of the equipment that perform many of the same functions. So, it’s not unusual for digital photographers to use lens filters for strictly creative enhancements and alterations to ‘standard’ photographs.