A fuel injection occurs when a special pump sends fuel under pressure from the fuel tank of a vehicle to the engine bay. In the engine bay, the fuel is distributed uniformly to each cylinder. The fuel thusly fired into the inlet manifold or inlet port via an injector that sprays the fuel in a mist-like form.
The purpose of fuel injections is to deliver the fuel seamlessly to the engine cylinders that need it. Fuel injections are able to do this with absolute control and precise timing. If your vehicle has a diesel engine, they use fuel injections by design. Petrol engines can also use direct injections where the fuel is delivered into the combustion chamber directly.