Issue with the Connect Line Tool

[Corey]

You are a legend! Haha, the clean-up command a long with everything we discussed earlier gave the successful edit!

While at one stage I somehow ran the circularize command and received a somewhat similar extrusion (after using a different restarted model, which for some reason didn't require a clean-up), hitting backspace actually deleted the entire thing! Which was because I rather foolishly had mistaken the tutorial for asking for 'D = 0' '100%'. This also gave me the strange square.

Thanks again, just for my own interest what does the clean-up command actually do?