Not sure if anybody gives you the answer to "what is tailscale?". So, this is my answer (hopefully it's correct and simple enough to understand).

Tailscale allows devices that can access the Internet (no matter how they access the Internet) to see each other.

To do that, you create a tailscale network for yourself, then connect your devices to that network, then your devices can see each other. Other devices that are connecting to the Internet but not to our tailscale network won't see your devices.

AI might explain it better :-) Don't know why I wanted to explain it.