You're not dense, you're just used to pump shotguns clearly.
The idea is that you load a round into the chamber FIRST while the BCG is back and the gun is empty. You then release the BCG, which lets you load additional rounds into the tube. This way, you always have a round in the chamber so you can use it if you need it. It's superior to a pump in that with pumps you have to load rounds into the tube, then pump, in order to accomplish the same thing. The m4 is faster getting a round in the chamber when you have an empty gun. If the bolt is closed, it always allows loading of more shells.
The setup is also designed so that you can change the shell that's in the chamber without affecting the shells in the mag tube. It works better than the pump way.
The small lever is there to reset the system on command. It allows the BCG to be locked back regardless of how whether the chamber is loaded, not loaded, rounds in the tube, no rounds in the tube, etc. On the next BCG cycle, the bolt will remain locked back, allowing you to manually load or change rounds if you desire.
Overall, it gives you much more options and flexibility, not to mention speed, over the standard pump system. It just takes some getting used to, as it is rather different.
Once you get good with it, you'll realize how awesome it is.