I can guess by the names that they are probably “virtual bridges”, but what exactly are they and, more importantly, why has Fedora 33 Workstation configured them without my knowledge? I have only recently noticed that they exist, and I don’t know how long they have existed.
That interface has been showing up for at least since version 30.
It is a virtual device and interface, and as has already been said is used by any VMs you might configure. I use bridging mode on my VMs and the virbr0 device so they have direct connection to the host machine, although on a different subnet. virbr0 resides as IP 192.168.122.1 on all my machines. The VM has access to the internet since the default config for the host is forwarding and NAT for traffic from the VM.