Filters are nodes that process an input to create a modified, "filtered" output, in a way you might be used to from other well-known imaging applications. The big difference with Material Filters is that they only work on one (sometimes more) inputs that do not represent a full material.
Filters are one of the most used major categories. Apart from the Atomic nodes category, no other category is as useful when building textures from scratch. Take the time to get to know each of the subcategories:
Expanded color, tone and value adjustments.
Advanced and expanded blending modes.
Advanced, high-quality blurs.
Expanded Channel-mixing nodes.
Texturing-oriented image effects, as well as "Layer Style"-type effects.
- Normal Map
Normal Map focused operations.
Nodes focused on fixing and improving tiling.
Advanced (mathematical) 2D-space transformation operations.