Variables are anything that might impact the outcome of your study. An operational definition describes exactly what the variables are and how they are measured within the context of your study. For example, if you were doing a study on the impact of sleep deprivation on driving performance, you would need to operationally define what you mean by sleep deprivation and driving performance.
In this example you might define sleep deprivation as getting less than seven hours of sleep at night and define driving performance as how well a participant does on a driving test.
What is the purpose of operationally defining variables? The main purpose is control. By understanding what your are measuring, you can control for it by holding the variable constant between all of the groups or manipulating it as an independent variable.