Simply put; a fuel injector is an electronically controlled valve that is either fully open or fully close. The period the valve is fully open is measured in millionths of a second. Its  purpose is to supply the engine with enough fuel and it is controlled via the engine management system.

Fuel Injectors have the finest tolerances of any mechanical part on the engine. They are very resilient and does not go bad often. In most cases the fuel injectors can be restored to original factory specification with proper fuel injector cleaning service techniques.