I do believe this refers to the mitochondrion, better know by its plural nomenclature mitochondria. Studies show that at some point in evolution, a eukaryote must've somehow "eaten" a prokaryote, but the prokaryote survived. Because of this prokaryote's beneficial characteristic of turning what it "eats" into energy, the eukaryote kept it as an organelle, forming a symbiotic relationship. As natural selection follows, this kind of eukaryote was much more fit, thus eventually taking over as the common structure of all eukaryotes. That prokaryote was the mitochondrion. However, since it was originally a different organism, it has it's own DNA, different from the actual eukaryote's genetic information.