Describe, in your own words, the purpose of an oxidizer in a liquid rocket engine.

What will be an ideal response?

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Ans: 

In normal combustion we need two things

Fuel

Oxidizer

1. Fuel: The thing that burns. This is is often a hydrocarbon, or other organic molecule. The simplest possible fuel is pure hydrogen gas.
2. Oxidizer: The molecule that accepts electrons. It turns out that combustion requires the fuel to be oxidized, that is, it donates electrons. So we need something to accept the electrons, and that's the oxidizer, which is then reduced.

Oxygen is a great oxidizer because it is so electronegative, which means it really wants to accept electrons. Only fluorine is more electronegative, and fluorine is a superb oxidizer: blow fluorine gas at nearly any substance and it bursts into flames.

Oxygen is in many ways a perfect oxidizer for supporting life: it doesn't set things on fire like fluorine, but otherwise can oxidize very many things. When you eat, oxygen is oxidizing the food (fuel) you ingested, to generate energy you need to live. So there is "combustion" going on inside.

But why do we need separate oxidizer in rocket engines, with in atmosphere there is plenty of Oxygen available but not in space. so, to make the combustion possible we should use oxidizer with the fuel for the rocket engines.