I’ll try to ELI5, if there’s something you don’t understand ask me.

Op has a home server where he’s running immich, that’s only accessible when he’s at home via the IP, so something like http://192.168.0.3:3000/, so he installed Tailscale on that server. Tailscale is a VPN (Virtual Private Network) that allows you to connect to your stuff remotely, it’s a nice way to do it because it is P2P (peer-to-peer) which means that in theory only he can access that network, whereas if he were using one of the many VPNs people use for other reasons, other people on the same VPN could access his server.

Ok, so now he can access his immich instance away from home, all he has to do is connect to the VPN on his phone or laptop and he’ll be able to access it with something like http://my_server:3000 since Tailscale adds a DNS (Domain Name System) which resolves the hostnames to whatever IP they have on the Tailscale network.

But if you want to give your family access it’s hard to explain to them that they need to connect to this VPN, so he rented a VPS (Virtual Private Server) on some company like DigitalOcean or Vultr and connected that machine to the Tailscale network. He probably also got a domain name from somewhere like namecheap, and pointed that domain name to his VPS. Só now he can access his VPS by using ssh user@myserver.com. Now all he needs to do is have something on the VPS which redirects everything that comes to a certain address into the Tailscale machine, Caddy is a nice way to do this, but the more traditional approach is ngnix, so if he puts Caddy on that VPS a config like this:

immich.myserver.com {
    handle {
        reverse_proxy my_server.tailscale.network.name:3000
    }
}

Then any requests that come to https://immich.myserver.com/ will get redirected to the home server via Tailscale.

It is a really nice setup, plus OP also added authentication and some other stuff to make it a bit more secure against attacks directly on immich.