A control variable is one which may, in addition to the independent variable, affect the outcome of the investigation and therefore has to be kept constant or at least monitored. If a variable should be controlled, but isn't, then the experiment will not be a fair test. If it is very hard to control something - such as the amount of light coming through a window on a day when clouds keep passing overhead - then you should monitor this variable as well. i.e. record the light level through the window when you also take your main measurements; that way you can see whether it has affected the outcome. e.g. Valerie and Vernon's experiment with the cylinder of putty's electrical resistance would have been a fair test if they had controlled the cross-sectional area. By cutting part of the putty off, they would have been able to investigate different lengths, with no effect from other variables.
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