NAT
Also known as Network address translation
Translating private internal IP addresses to a public one so many devices can share a single internet address.
NAT is why a whole office can operate behind one public address. It also means inbound connections have nowhere to go by default, which provides incidental protection though it was never designed as a security control.
It complicates anything needing inbound access, including site-to-site VPN when one end sits behind a carrier-controlled NAT it does not manage.
Where this comes up
NAT sits inside our networks and infrastructure work. If you are trying to work out what this means for your own environment rather than in the abstract, that is the page worth reading next, and a short call will get you a straight answer faster than either.