Mainly STUN and TURN servers to allow NAT traversal without having to configure port forwarding and leave your server exposed to the internet.
It’ll use those servers to setup a peer-to-peer connection which at that point you are streaming directly to clients.
If you want to setup a VPN for your users, or open/forward public ports to your server then you do not need to pay.
Also, fuck the profs for choosing to use it, and make it a requirement for a course.