For two molecules to react, they must first come into contact with each other. This contact can be considered a "collision." The more mobile the molecules are, the more likely they are to collide. In addition, the closer the molecules are together, the more likely they are to collide.
In the following drawings, the molecules are closer together in the picture on the right than they are in the picture on the left. The molecules are more likely to collide and react in the picture on the right.
The two figures above might be described in terms of population density. Both drawings appear to offer the same amount of space, but are inhabited by different amounts of molecules. The difference is much like the difference between human population densities in various locations around the world. Some places, such as Mexico City or Tokyo, are very crowded; they have high population densities. Some places, such as the Australian Outback or the Canadian Arctic, have low population densities.