It's a website that has compiled existing open source emulators that are capable of running old versions of MacOS and NeXTStep, to WebAssembly so they run inside a modern browser.

They have paired that with disk images of these OSes and a lot of tweaking, so that when you go to the website, you select the version of the OS you want to play with, and _it just runs_ in the browser.

Instant old Mac/NeXT experience, no separate download or software to install.