It's pretty much exactly what it says it is. Solo instruments recorded well, playing renditions of familiar tunes, if somewhat simplified. It pretty much sounds like someone in a guitar center going through the different sounds on a guitar/electric piano/saxophone/trumpet/steel drums/what-have-you, noodling around in an attempt to capture what was originally played in an ensemble format. Except the musician really knows the material. You have to really love the sound of the instrument to be able to listen to it for 36+ minutes, which I happen to (at least, the electric piano and the accordion ones). Take, for example, the electric piano version of Peter Frampton Comes Alive -- it's almost half as short as the album. And they don't add distortion, so it's not so much produced as performed straight. I'm tempted to grab one of the electric piano ones, just so I can sample the shit out of it. I'm listening on Tidal, on which the artist is listed as "Solo Sounds". Some of them are pretty brave -- Miles Davis' Kind of Blue!
I should put it on shuffle.