RTP allows other people that don't have RPG Maker play your game. It's a larger download for those people, though.
You can find it in your rpg2003 folder. If you don't know where that folder is, follow these instructions: First, go to "My Computer". Then go to whatever your computer is called/what you save your files under. Then, open up "program files". Then, find and open "rpg 2003". In there you'll find the RTP folder. In there, you find EVERYTHING in RPG Maker 2003.