Fuel and oxidizer, together they create propellant, which can be used to launch a rocket, but for the finest outcomes, you need to discover the correct ratio. Too much of one or the other, and your exhaust velocity initiates to fall. Less exhaust velocity means less push, and less push means you will not go to the space today.
In this video from BBC's Bang Goes the Theory, the idea of fuel/oxidizer ratios is explored by means of soda bottles, chemical reactions, and slow-motion HD video, all to excessive result. You'll want to watch this one full screen, HD, etc. Now sit back and study a minute rocket science.