A signal is a mechanical or electrical device erected beside a railway line to pass information relating to the state of the line ahead to train/engine drivers

Railroad engineer

A railroad engineer, locomotive engineer, train operator, train driver or engine driver is a person who drives a train on a railroad...

. The driver interprets the signal's indication and acts accordingly. Typically, a signal might inform the driver of the speed at which the train may safely proceed or it may instruct the driver to stop.

Originally, signals displayed simple stop/proceed indications.