Imagine that we have a container in which we have two types of reacting particle,  A and B.

There are a number of possible scenarios;

1. A and B do not collide
2. A and B do collide but they "bounce off each other"
3. A and B collide with each other with sufficient energy and a new material is formed.

INS [Coll_CI_01_ANI]

When A and B react with each other they make a molecule which has the formula A-B and the reaction is successful.

This what is meant by collision theory.

To increase the amount of the new material product  (A-B) we can manipulate the conditions so we get more successful collisions.

If we raise the temperature the energy of the particles will increase. The number of particles which have sufficient energy to react therefore also increases and we get more successful collisions so we get more product.

We can also get more successful collisions by squashing A and B together by putting the same number of particles into a smaller space. This can be done by either reducing the volume  of the container (increasing the pressure) or by adding more A and B in the same space (increasing the concentration). Either way we will be increasing the chance of A and B colliding therefore increasing the chances of more successful collisions.

The key to affecting the rate of reaction is found by manipulating the conditions.