In an experiment, there are several factors called variables that are likely to affect the outcome of the experiment. When you do an experiment, it is very important to control all the variables except for one, which you will change in varying amounts. The variable you change is called the experimental variable. The variables you control, or do not allow to change are called contol cariables. You control the "control" variables because you want to be able to say the reason the experiment came out the way it did was because of the changes made in the experimental variable.
For example, you may do an experiment that hyopothesizes about the amount of sunlight having an affect on the growth of plants. You would change the amount of sunlight several different plants receive as an experimental variable. You would keep all the other variables that are controls the same, such as amount of water, fertilizer, soil type and pots the plants are in. If you do not control the control variables, you can not accurately say your experimental variable was the reason you had a change in the outcome of the experiment.