On the wiki, the "what is retrodeck" explains it, but it assumes you're familiar with similar software. It lists some Linux OSs geared towards emulation. I think if you want to get the gist of what it is, look at Batocera.
But yeah, it seems the general idea is it's a frontend to put all the emulators in one UI.
I wish one of these projects would bring in changes from some fork of emulation station I can't track down.
I had a handheld that came with some emuelec-based image. That in turn had some version of emulation station that was freaking AWESOME. Had way way more features, including the ability to configure a bunch of things like whether rewind support was enabled, etc. When I moved to the ES:DE one I thought it was a huge downgrade, then realized it's pretty much the main one everyone uses. I don't get why they haven't brought in these other features.
I also haven't been able to find the fork; I should go look again.