Unloader checks for unused dimensions every 30 seconds (600 ticks) to unload.
An unused dimension is classified by the dimension having no chunks or entities loaded. Once it finds one, it tells forge to unload it.
This can free up tick time and RAM for your server or client! (Although it's not a whole lot... Don't expect it to magically fix everything.)
Also helps with TPS commands so you're not getting a bunch of dimensions appearing that you don't need to see.
This can be installed on server or client, but if you're on a server without this mod, you won't get the benefits.