Bottom bracket removal, what is this part?
It's a bottom bracket cup. There are specialty tools you can use to remove it such as the HCW-11 on this page, or if it isn't rusted in, you can probably accomplish the same with an adjustable crescent wrench.  Under there are some ball bearings, so make sure you don't lose any when you open it up.  The bearings should be in a cage, which would stop you from losing them, but with an older and worn bike such as the one shown in the picture, the bearing cage may be broken.