A brush or paint filter option is a part of the Partial Correction Tool for applying corrections to irregular regions.
Some examples of why you might want to use the Partial Correction Tool is to increase brightness on some subjects darkened by backlighting, or fine tuning the white balance on some subjects seemingly brightened by reflected light.
Selecting an Irregular Region to Apply Partial Correction
By using a paint or brush-like tool, you can make more granular selections, which is particularly important when objects are complex or uneven in shape.
Brush Correction Filter in the Partial Correction Tool