Generally speaking, when testing a hypothesis by performing a lab experiment you run at least one test run and at least one control run. The control part of the experiment is running the experiment without making the change that you are doing to test the hypothesis. And correspondingly, the test part of the experiment contains the change that you are testing the hypothesis with. For example, if you are hypothesizing that a particular plant will grow slower at colder temperatures, then the control run will be growing the plant at normal, warmer temperatures (not testing the hypothesis) and the test run will be growing the plant at colder temperatures (making the change necessary to test the hypothesis). The control run makes sure that the background experimental conditions are being set up appropriately so that you can be sure that the results from your test run are reliable.