It's a theoretical concept. I haven't seen an actual MAV example that demonstrates this but I'm woefully lacking in following MAV development
so take my comment with a grain of salt.
Conceptually a fixed wing MAV will have a harder time maneuvering in confined indoors spaces because of the airspeed or wing span & surface area needed for very low speed flight. Contrast to that of a small bird or insect that easily maneuvers in confined spaces at very low forward airspeeds and even hovers. So the idea is that a MAV based on the same principles flight principles of birds or insects would be more agile in that respect vs. a conventional wing.
Here are a couple of papers on the topic:
Flapping flight is quite complicated to describe. But consider the following difference to expound a bit more on the concept above.
Fixed wing aircraft get their primary lift generation as function of forward airspeed and amount of wing span & surface area. (Yes angle of attack is a part of it but the amount of aoa needed is a function of the airspeed.) For MAV flying in confined spaces very slow speeds are needed to avoid running into obstacles. The only way to do that with a fixed wing MAV to reduce it's flyable forward airspeed is to increase the size of the wing. As you can see this is a self defeating proposition - we have to make a larger MAV to fly in smaller spaces. Hmmm.
Flapping wings get their lift not from forward airspeed of the aircraft but from flapping motion of the wing. Because of this there is less reliance on forward airspeed to generate lift and the amount of lift (and thrust) generated can be controlled by how fast the wings beat. This conceptually solves the issue for a fixed wing MAV trying to fly at very slow airspeeds or even hover.