Dr. Pimple Popper squeezes and pops all sorts of skin blemishes, like zits and cysts and lipomas (oh, my!), but in this video she goes back to basics, taking care of a patient's big back blackheads. This is a back Dr. Sandra Lee (which is Dr. Pimple Popper's real name) has seen before, and it's filled back up with blackheads—the gift that keeps on giving. There's even a dilated pore of Winer (which is basically just a really big blackhead) in there for all you fanatics.
According to Dr. Pimple Popper's YouTube page, a blackhead (which is also sometimes called an open comedo, as opposed to a pimple, which is a closed comedo) is clogged skin pore that's open to the air. The dark stuff inside them isn't dirt, it's the same keratin and sebum that fill pimples, only it's gotten darker from being exposed to the air.