STUN (Session Traversal Utilities for NAT) is not NAT by itself but it is quite important in today’s world. Whether it is controlling things like cheap remote camera’s, remote controlled smart thermostats, cloud controlled access points and remote computers with TeamViewer or Quick Assist.
Simply explained, STUN is a technique where there’s actually a server or servers in between both the clients (for intance your smartphone and the remote smart thermostat) to which they both connect. The server is basically the middle man that receives commands and data from one client and passes that to the other as if the devices were actually connected together.
Using STUN doesn’t require port-forwarding at all. Though, STUN can come with security issues if the server would be compromised by remote attackers and has access to any of the clients connected to it.