Edevore's right; a ROM (or "ROM image") is a file that holds all of the data from a game cartridge (except for the save games, which are not "read only"). It's basically a game stored as a single file, which can be played via emulator.
Further reading:
http://wikipedia.org/wiki/ROM_imageI was confused by the name when I first got into emulation, and I was a high schooler! So, don't feel too silly.
