Fuel and oxidizer. Together they make propellant, which can be used to launch a rocket – but for the best results, you need to find the right ratio. Too much of one or the other, and your exhaust velocity begins to drop. Less exhaust velocity means less thrust — and less thrust means you will not go to space today.
In this clip from BBC's Bang Goes the Theory, the concept of fuel/oxidizer ratios is explored using soda bottles, chemical reactions, and slow-motion HD video, all to great effect. You'll want to watch this one full screen, HD, etc. Now sit back, relax, and learn a little rocket science.