A hazard is anything that causes you to change the speed and/or direction of your vehicle.
For example:
A cyclist is a hazard because a driver will have to alter their direction to go around them. They will also, potentially, have to alter their speed.
A bend in a road can be a hazard.
If the road conditions are wet - the road has become a hazard.
If a pedestrian walks towards the road between 2 cars - there is a potential hazard.