Fidgeting simply means that you're incapable of keeping still. Like nail biting and other so-called "nervous habits," it can be a way to fight boredom, expend excess energy or relieve stress.
This bad habit isn't all bad, though; some researchers believe that fidgeting reduces levels of the stress hormone cortisol. Since cortisol can negatively affect learning, fidgeting during a test might actually make you perform better on it. Because the parts of the brain that control movement and speech are in the same area, fidgeting or moving may also help you formulate a thought before saying it out loud [source: BBC]. You might even burn an extra 350 calories from fidgeting throughout the day [source: Grady]. But fidgeting is one of those bad habits that can have a social impact, especially if it is constant, loud or distracting.
Usually when someone calls attention to fidgeting, the person will stop doing it, or at least find a way to be unobtrusive about it. But if he can't stop, the fidgeting is constant, or he seems to be involuntarily twitching or making noises, there could be something more serious at work. Extreme fidgeting is a possible symptom of several conditions, including attention deficit hyperactivity disorder (ADHD), bipolar disorder and Tourette's syndrome. Professional help is necessary in those cases [sources: Ungar, NHS].