The control group is the group where all the factors are set to 0 effect, also never change over time or have a controlled change, or having a median effect.
For example, if you want to test if green light is better for plants or Violet light.
You already know that Sun light is the optimum condition for plants growth.
So you would have 3 sets of plants, one that will get only green light, another to get only violet light, and the third one is to get sunlight. This third set is your control group.
So it actually shows the normal effect, and make the comparison more reliable and notable with the other groups tested in different conditions.
Hope this helps!
