A fuel rail is essentially a pipe (usually resembling a rail) used to deliver fuel

Fuel

Fuel is any material that stores energy that can later be extracted to perform mechanical work in a controlled manner. Most fuels used by humans undergo combustion, a redox reaction in which a combustible substance releases energy after it ignites and reacts with the oxygen in the air...

 to individual fuel injectors on internal combustion engines. It is designed to have a pocket or seat for each injector as well as an inlet for a fuel supply. Some fuel rails also incorporate an attached fuel pressure regulator

Regulator (automatic control)

In automatic control, a regulator is a device which has the function of maintaining a designated characteristic. It performs the activity of managing or maintaining a range of values in a machine. The measurable property of a device is managed closely by specified conditions or an advance set...

. Fuel rails are used on engines with multi-point fuel injection systems, although some multi-point systems use a fuel distributor with individual pipes or tubes to feed each injector.