This is a screenshot of the inventory screen for the One Little Rat mod for Sonic Robo Blast 2. This is actually somewhat outdated, but nothing has really notably changed since I took this screenshot other than support for further items on the right panel.

The white box in the left panel is the selection cursor, which can select anything within the left or right panel. If you press the Spin, Custom 1, Custom 2 keys/buttons, you an swap the selected item with what is in the slot for that key (shown at the top). You can't do this with items in the right panel, but you can sometimes interact with them in other ways. For example, if you have the Converter selected, you can toggle which mode it is in (not spoiling what the Converter is here or what the modes are) by pressing one of those three keys/buttons.

As you likely guessed, the bottom panel describes what is currently selected. The blue meter on the top right is energy, while the pink meter is health. Last, but not least, there is a reason the rings are listed exclusively here, rather than outside of the inventory screen as well. That is because the protagonist is a little rat and isn't awesome enough to protect itself with rings like Sonic and his friends are. However, if you collect enough rings (right now, I have it at every 100, but I may increase the required amount), you get a permanent max health boost.