When you edit features in a topology, the topology tracks where changes have been made. These places are called dirty areas because a topology rule could potentially have been violated by the edits, but the error, if it exists, cannot be found until the dirty area is validated again. When you validate the topology again, it just checks the dirty areas. You can see the areas that have been edited by showing the dirty areas in the topology layer.

The dirty areas cover the features that you edited. Dirty areas optimize the validation process, as only these parts of the data must be checked for topology errors.

After you validate the topology around a dirty area, the dirty area is removed.