MTU stands for Maximum Transmission Unit.
When data is sent over a network, it must adhere to the maximum transmission unit available. This means that if we are trying to send a chunk of data bigger than the MTU size, it will be fragmented into smaller chunks and sent in multiple packets over the network (or not sent at all).
To efficiently send media over the network in real time, our goal would be not to have the packets we send fragmented by the network or dropped due to too small MTU size, which is why we end up packetizing the media frames prior to sending them.
Different routers and switches in the network may have different MTU size configured in them, so figuring out what is the effective MTU size of a given network communication path can be tricky.
In most networks, it is usually assumed that the MTU size is around the 1,500 bytes mark. The approach WebRTC took here is to set an MTU size of around 1,200 bytes and use it for its packetization calculations (give or take a few bytes). Using this lower value makes sure WebRTC will work well on most network configurations (including scenarios where packets are further wrapped by VPN tunneling bytes or additional layers of encryption).