Controls are variables in a scientific experiment that are not allowed to change, they are controlled to be the same values in both the experimental group and the control group. Let's say you are testing to see if the amount of sunlight a plant receives has an effect on it's growth. The variable you would change between the experimental group of plants and the control group of plants would be the amount of sunlight the experimental group got. All the other variables, such as type of soil, amount of water, ventilation, would be kept the same. You want to control all the other variables because we want to test one variable at a time to see if it causes the effect we think it causes. Failure to control all the other variables could give incorrect analyses which would support an incorrect conclusion.