An emulator makes it possible to run software on your computer that was intended for a different kind of computer (or rather a different microprocessor). The emulator itself is a piece of software that mimics the microprocessor and all subsystems of the computersystem you want to emulate.
There's probably an emulator for every computer out there, so on your Windows PC you can run software for the Commodore 64, MSX 2, Atari ST, Commodore Amiga, Apple Macintosh, Sony Playstation, Sega Saturn, etc. etc.