In the early nineties, Sega released the Sega CD, a.k.a. Mega CD, an add-on that attached a CD-ROM drive to the Mega Drive a.k.a. Genesis. Its primary or at least most conspicuous use was for full-motion video games that were appearing at that time.
Playing full-motion video from CD really needs pretty hefty compression if you want more than a few minutes of it. The Genesis CPU was a 68000, which is far from capable of doing that sort of hefty decompression in real-time. The machine was not designed for CD-ROM or FMV, so presumably did not ship with the kind of specialized hardware that could do it.
How exactly was the decompression done?