What does controlling variables mean in science?

When you are performing an experiment, you want to change only one variable at a time. Let us say that you would like to know how different temperature affect the speed at which mushrooms grow. If you only alter the temperature then the interpretation of the experimental results will be clear. If you alter the temperature and also alter the amount of water (for example) then the interpretation of the results will no longer be clear, since the results might have been caused by the temperature, or by the water, or even by the combined effect of the temperature and water. Therefore, you control your variables.