When you install Tailscale on a device, that device is now a part of a virtual private network, and gets its own IP, usually starting with 100.*
So if you usually access your Jellyfin instance 192.168.0.1:8096, and you install Tailscale on your server and a client device, check which IP Tailscale assigned your server and replace 192.168.0.1 with that IP. Make sure to keep the port in the url (the thing after the colon. Jellyfin’s default webui port is 8096).
The main benefit here is not having to open a port. You can still access the port that Jellyfin’s webui is behind without exposing it to the internet.