A hazard is a source of danger. When driving, it is something that forces you to change your speed (usually to slow down) or change your steering (usually to steer around it) to avoid a collision.
Real and potential hazards
A real hazard is something that you have to react to. Examples of real hazards are: a stop light, a car moving slowly or stopping in your lane, a curve that requires you to slow down, a pedestrian or vehicle blocking you.
A potential hazard is something that mIght happen and you may have to react to: a car signalling the intention to change lanes, a car on the shoulder that might pull out, a pedestrian who might cross the road or a traffic light that might change.
These two types of hazards will be referred to frequently in the following sections.