Install the client on two devices and log in with a Gmail account. That's it.

They'll be assigned IPs and can reach each other.

(I'm a co-founder of Tailscale.)

That's really cool. Thank you.

what's local networking got to do with gmail?

There needs to be a mechanism to distribute keys to nodes / authenticate nodes / associate nodes.

Tailscale offers a centralized server to allow nodes to provision themselves. It bootstraps that trust/account system of of logging in with some account, such as via a google oauth login.

Right! That makes sense, thanks.