I really want to write a book on this eventually, but I haven't found a good single resource myself :( I have done some talks trying to explain WebRTC. Happy to answer any questions
It standardizes a set of conventions that web browsers could follow to a) solve the issue of NAT traversal (i.e. opening ports in consumer routers and firewalls), b) send video and audio (and possibly also arbitrary data), directly between web browsers, and c) possibly adapt the video quality to the conditions of the network, in an autonomous and automatic way.
That's the bird's-eye view of it.
Some keywords to expand on this: "a)" is done with the ICE protocol, "b)" is done with plain old SRTP, and "c)" is done with algorithms called REMB or Transport-CC.
