Assume all your machines have Internet access, which they probably do these days. Instead of having to create firewall rules and routes and that whole mess in order to connect to a machine on "the inside" somewhere, the machine reaches out to the Internet and creates a tunnel, and you connect to the machine through that tunnel.

Isn't that a VPN?

Yes... but a painless and damn near zeroconf one. It's wireguard underneath.

i use it to access my NAS/network drive when im away from home.

when im at home i can access it no problem since im connected to my home network and so is the NAS. when im away its not possible to connect to it because im on a completely different network.

with tailscale installed on each device makes it seem like im still on my home network even when im away.

So its a bit like a VPN? Except that you can join specific devices from a multitude of networks?