virbr are bridge interfaces. They are virtual in that there's no network interface card associated to them. Their role is to act like a real bridge or switch, that is switch packets (at layer 2) between the interfaces (real or other) that are attached to it just like a real ethernet switch would.

The fact that their name starts with vir doesn't make them any different from any other bridge interface, it's just that those have been created by libvirt which reserves that name space for bridge interfaces