What does it mean to have a controlled scintific expiriment?

In a controlled experiment, the experimenter creates two groups. One is the control group, and no treatment is given to that group...it remains unmodified. The other is called the experimental group, and that group has a modified treatment relative to the context of the experiment.

For example, if you wanted to determine whether a chemical X would enhance or hinder the growth of a particular plant, you would set aside a control group that would receive standard water and sunlight, and you would create an experimental group that would get water, sunlight, and chemical X.

This way, you isolate all other variables but the addition of chemical X, and you can determine whether or not it has an effect on the variable being tested.