A railway signal or just signal is a mechanical or electrical machine that gives train drivers or engineers information about the state of the line ahead. This way, they know whether the train must stop or may continue. It can also tell what speed the train may go.
In Britain, modern signalling uses a system similar to Traffic Lights. The signals are coloured lights mounted on a pole. A red signal means 'do not go any further'. A yellow signal gives a warning to the driver that the next signal is showing a red. Some signals can show two yellow lights, known as a "double yellow" and are warnings to the driver that the next signal is showing a yellow. A green signal means 'you can continue'.
On older lines semaphore signals are still used. These are poles with a coloured bar at the top, which can be moved so that it is either horizontal, or at an angle. When a red bar is horizontal this means that the train must stop, and when it is pointing diagonally up or down it means that it's safe for the driver to continue. There are also signals with yellow bars which give warnings in the same way as yellow lights on newer signals.