Its a program that runs console games on your computer. Such as mario, or legend of zelda, or final fantasy. Although, I have only gotten one to work for final fantasy, and, mario
Do note, some emulators won't work, or are simply demos that don't run any standalone games.
Zophars domain won't give you emulators though that are false, so you can trust them to give you real ones. Read the descriptions next to each one to find the best.