It stops internal bridging within the AP driver. Devices that are connected to the same wifi AP (all of them would be guests) should not link to each other. As noted, whether this will actually work varies. And if you have a layer 2 connection (wired, mesh, or WDS) between two APs on the same guest network, guests on one AP could still link to those on the other AP.
Isolating the guest network from the main network is done in the firewall.