To expand a little bit: it provides an (almost) zero-configuration way to set up a private 'layer 2' network that you can connect your home server to, and any other machines that you want to be able to connect to it or to each other. It handles NAT traversal completely transparently.
In practice, it means that if I have a (say) NFS server connected to a Zerotier network I control, I can connect to it transparently from anywhere from another machine on that network, no matter what NATs / firewalls either machine is behind, even if they change. Perfect for phones, roving laptops, etc. I've gone to a model where I do most of my development (over mosh/tmux) on my home machine, from wherever I happen to be.
https://github.com/n-t-roff/sc (active fork)
https://en.wikipedia.org/wiki/Sc_(spreadsheet_calculator)
https://manpages.ubuntu.com/manpages/resolute/en/man1/sc-im....
Installable on mac and unix as ‘sc-im’