These are your mute and solo controls. First, we'll need to define a subgroup. Each channel strip eventually gets summed together and put on the mix buss where it gets routed to your monitors and your mixdown deck. Between the channel strips and the mix buss is often a collection of subgroups. These subgroups are a way to organize your sounds into logical groupings for further processing.
Subgroups are useful for combining tracks that belong together.
Likewise, you may also group all background vocals onto a subgroup so they may be processed together as a whole. Depending on the manufacturer, there may be stereo or mono returns, or a combination of both. The Spirit mixer that I own has 4 stereo returns. They're quite useful, nonetheless, because they allow you to connect your effects boxes to your mixer without using up your normal channel strips. The monitoring section of the board is essentially the master volume for the entire mixer.