A Solo Frame is almost exactly the same kind of a container as a normal Frame that can hold other widgets and other frames inside. However, the difference with Solo Frame is that it treats any Buttons inside it a bit differently by allowing only one button to be depressed at a time. For example, consider you have Button A and Button B inside a Solo Frame with Button A currently depressed. Next, you click button Button B which automatically results in Button A being released, leaving now only Button B depressed.
Solo Frames have no configurable options whatsoever. You can only change their styling & placement options just like all other Virtual Console widgets.