What is collision theory?

I suppose that you are talking reactions here.

Collision theory basically says that the more densely packed molecules are, the more likely a chemical reaction will occur. This is because chemical reactions occur BY CHANCE. As in, the reaction occurs if and only if the reactants collide.

imagine this: you have mice, running all over a deserted mall. And to reproduce, one gender must find another. So, is it more likely for the mice to reproduce in a mall filled with mice, or in a mall with much fewer mice?

It's the same concept, except substitute the mice for the reactants, and the baby mice for the products. Oh, and also, make sure the mice die after reproduction...that's how the reactants work: they're consumed in the reaction