It allows you to move tabs into named groups. Only tabs from the active group are shown in the tab bar, and you can switch between groups using an icon in the toolbar.
As a pre-Quantum Tab Groups user, I find that Simple Tab Groups is considerably more stable and has a slightly better UX than the old support. So A+ for that. I still feel like there's some missing "secret sauce" here, but I haven't had the time to really delve into the problem to suss out what that might be.
I do love that tab groups are malleable to the user's concept of organization, not some artificial notion such as grouping by website.
Unfortunately Simple Tab Groups (or maybe some internal feature it uses, probably hidden tabs) somehow leak resources and end up eating all the RAM for me.
The end of Panorama (or Tab Groups as known nowdays) was a disaster for me, as I don't hesitate having over 3k tabs open.
When I open up a tab group with many tabs in it, the extension loads all of them up (snoozed) in a way that takes a little bit of time to complete, but it works well.
It allows you to move tabs into named groups. Only tabs from the active group are shown in the tab bar, and you can switch between groups using an icon in the toolbar.