mDNS stands for Multicast DNS. it is defined in IETF RFC 6762.

mDNS is meant to deal with having names for machines on local networks without needing to register them on DNS servers. This is especially useful when there are no DNS servers you can control – think of a home with a couple of devices who need to interact locally without going to the internet – Chromecast and network printers are some good examples.

In the context of WebRTC, mDNS has been introduced to protect against the JavaScript application accessing the local IP addresses that are exchanged during ICE negotiation. This is achieved by the browser replacing its local IP addresses with random mDNS ones that it registers on the local network.