Hey Az,
A chronograph in its simplest form is a speedometer for bullets. What you can get today at fairly reasonable prices, however, is a lot more than just a speedometer. Basically, a chronograph is a computer than can measure the speed of a bullet as it passes through two screens which are usually located a short distance from the muzzle of the gun. The computer is connected to the two screens which sense when the bullet passes through. Based on the lapse of time for a bullet to pass through the two screens (the screens are set at a specific distance from each other), the computer calculates the velocity of the bullet at that point. Chronographs used to be an arm and a leg thirty years ago, but today you can get one for $100 to $200, and it will record and average strings of shots for you.
Best wishes,
Dave Wile