Hazard warning line at a junction
An approaching hazard in the road can come in many forms such as a closed junction (See junctions for further information on closed junctions) or a bend in the road. An approaching road hazard may or may not have a hazard warning sign.
In the diagram, the white line in the centre of the road changes from the standard broken white line into a hazard warning line with the white line lengthening and the gaps shortening.
This is used to inform the driver of a hazard being the junction, up ahead. A driver who doesn't notice the hazard warning lines or is unfamiliar with their meanings will have less time to react and prepare when they see the junction.
If you are not confident with road lines and markings, taking driving lessons with a qualified driving instructor will help. Ask them to explain the various lines and markings as you drive until you become confident.