A brush variant is either a cached or saved variation of an existing brush.
When you choose a brush from the brush library and select variant> restore default variant from the brushes palette menu, the chosen brush is then in its original default state ie. all of the settings are as they were when the brush was originally saved.
In Painter 7, if you then alter any of the settings for the above brush, you are then creating a variant of it. The new brush settings are not permanently saved as a new brush until you choose variant > save variant from the brushes menu and either rename it, or keep the name the same (if you want to replace the default brush settings permanently).
If you have renamed the variant and saved it, you will then need to go back to the brush from which the variant was created from and choose variant> restore default variant.
Painter 7 creates a cached variant file in the same brushes folder as the original whenever you change a brush setting from default value. The file is automatically given the same name as the original brush, but also has a 'c_' prefix. The cached 'c_' version is deleted when you choose variant> restore default variant.
If you have changed any of the default brush settings (even simply the brush size) and then quit Painter, next time painter is launched the changed settings will be used rather than the default settings until the variant > restore default variant is selected.