These are the nozzles that inject fuel into the engine-
one for each cylinder. They are mounted on the top
of the engine, on the intake manifold. They are fed by
a common fuel tube, know as a fuel rail. There are electrical connections on the top of each one, wired back to a computor that measures flow and mixture.
Unless you are a skillled mechanic, do not attempt to
mess with this system. Only a reputable garage would
have the right equipment to diagnose and replace any
injectors that are found to be defective.