A "ROM" in this instance is pretty much all the code and data needed by the emulator to run it as a calculator. Since providing such ROM is "legally questionable", you can obtain your own if you have a TI-83 Plus or Ti-83 Plus SE graphing calculator and some linking software that'll let you do a "ROM dump". TiLP is a good PC program to use for that purpose (if you can get it to work). Please don't ask for ROMs here, as I believe it's against the TOS to do so (among other things), but I believe it's okay to say how you can get your own copy legitimately.
