Zerotier is something I struggle to sumarise. The best way I can explain it is a mesh VPN that requires no setup on routers or port forwarding. It overlays a 'lan' if you will that you can connect servers to from anywhere. If possible it will pass traffic from host to host, but if that is not doable, it will bounce it past their servers.

Zerotier promise encryption, so I guess there's quite a bit of trust. They are Opensource so you can look in to their code if you like

All traffic is automatically end-to-end encrypted using keys only you control. Access to virtual networks is controlled by certificates.
ZeroTier, Inc.
Directly Connecting the World’s Devices with Universal Software Defined Networking - ZeroTier, Inc.
Zerotier Github

You will need to create an account and install it on both machines.

Create an account at my.zerotier.com/

Once your account is creates, click Networks at the top and then + Create a Network

You will see one appear under the Your Networks section  

Click the ID of the network and pick the network range you want. You NEED it to not be used anywhere else!

A note to my homies who hage CGnat, make sure that you check their range before you pick one, as well as your own internal range

Once that is done, you will need to install Zerotier on the machines you want to connect to each other.

If you trust SSL on linux:

curl -s https://install.zerotier.com | sudo bash

Else, Zerotier have good instructions here

Now we need to add the server's to the network!

sudo zerotier-cli join <network ID>

If it returns with 200 it's happy days. Wait a few minutes and you should see the first server show up in the management page. Add a name to it so you know what's what.

Rinse and repeat for all the other devices.

If you want your friends to be able to join your locally hosted game server, or view an internal webpage and you don't fancy setting up a reverse proxy then you can get them to join your zerotier network.

It works the same way a lan does, except it spans over great distances and can be used (I don't suggest it) as a VPN