Chronographs are devices for measuring the speed of a pellet fired from an airgun. In the UK airguns need to be kept below a certain power (12ft/lb) before an FAC license is required.
Chronographs are very much required if you want to tune and maintain your own gun. Most airgun ranges will test your rifle to check it is within the legal limits for use at their range. Not only this, but chronographs are helpful tools for analysing how fast different pellets fire from your airgun. They can also help determining the optimum fill pressure of your PCP which can help stop overfilling.