What does it mean to control a variable?

Question

Does it mean that you set limitations? Thanks.

Answer 1

It means that you design things so that the variable cannot influence experimental results.  It can be inherent to the experiment, or it can be part of your analysis.

So if I control for gender during analysis, I might, for instance, only compare participants of the same gender to one another when doing statistical tests, etc.

An example of control inherent to the experiment would be testing a drug only on patients who have had a certain type of cancer for exactly 3 months - you would be controlling for diagnosis.