Anybody who has used a manual camera might have come across the term photographic filter or may have actually used one. The most common being the UV/Protective filter, but there is more to it than just protection. It can help in altering the light, reducing reflections, enhancing colors etc.
As this post explains:
Filters to your camera are what sunglasses are to your eyes.
Natural/landscape photography enthusiasts particularly might have more knowledge about the different kinds of lenses available. For others, here is a quick Photography Filter Cheat Sheet from Zippi. Explaining the most common types of camera filters and its uses.