Detects your NAT type to judge whether P2P gaming, voice calls and remote streaming can connect directly. IPv4 and IPv6 are checked separately.
The check uses STUN servers to observe your outbound address mapping: whether the mapping changes per destination and whether ports are predictable. That distinguishes open, cone (full / restricted / port-restricted) and symmetric NAT.
With symmetric NAT two peers can rarely punch through and usually need a relay; open or full-cone NAT connects most easily. If a game is stuck on "connecting", this is usually the reason.