What is Pokemon Sapphire ROM?

A ROM is a version of a game that is able to be played on a computer. Say a person wants to play Pokemon Sapphire on their computer. They first download a gameboy advance emulator. This is basically a program that allows you to play games written for a GBA on a PC (it "emulates" the original platform). They then download the Pokemon Sapphire ROM, which can then be played through the emulator.