What’s an emulator?
An emulator is software which lets your computer pretend to be a different sort of computer.
It will allow a Raspberry Pi Zero to run software originally designed for the Sega Mega Drive, or Nintendo N64, old DOS-based PCs, and so on. Emulators aren’t without their problems, though. It’s nearly impossible to perfectly recreate a games console in software. Keep in mind that older games may have bugs ranging from minor sound and graphical glitches to full-blown crashes.