In scientific terms, the word control means to create a group that experiences the same conditions as a test group during a study, without being exposed to the experimental factor or treatment that the test group is. In doing this, scientists can reduce the chance that an external factor is producing the experiment results they experience.
When performing scientific tests, scientists cannot prevent other factors from influencing their results. They can, however, use a control group to understand those factors better. This is particularly important when studying drugs and medical treatments, as the test subjects' physiology may impact the outcomes.
Scientists therefore need to create two groups of people with similar characteristics being tested in the same environment to measure external influences and analyze them alongside the potential effects of a treatment. For example, when testing a diabetes medication, they may want to have people of the same age, weight and general health status to ensure such factors are not influencing the treatment's effects. If there are noticeable differences between the two groups, there is a chance the drug is creating the difference.
When using control factors, scientists are able to ensure tests are fair. Another example would be making a cake that only differs in terms of one ingredient to see if it causes an allergic reaction. If scientists did not keep all the other ingredients the same while attempting to prove that the ingredient they focus on causes the reaction, they could not fairly say what the cause is.