Active recovery is the idea of "recovering" from a more difficult part of a workout, while still moving/exerting some effort.
For example, after doing a 100 all-out, you may immediately push off again for a 50 easy. Because you aren't resting at the wall, but are still catching your breath by swimming easy, this is called active recovery.
This type of recovery is meant to help improve your body's response to lactic acid (produced during anaerobic exercise).