A hazard is anything that could encroach upon your space while you are driving, and cause a risky situation. It's true that almost anything might seem as though it could fit into this category (pedestrians on the footpath, farm animals behind a fence) but you have to use your judgement and consider the likelihood that the things you observe could really come into your path and affect your driving. We're only interested in you clicking on those, but great work if you're noticing more - the more you notice about your surroundings the better.
For example, an oncoming car in the distance isn't yet a hazard. But if that car is showing signs that it's about to turn in front of you, it becomes something to watch out for.
A pedestrian safely on the footpath isn't a hazard. But if they're doing something risky near the road or are about to cross over, then you've got to identify them as a hazard.
If you click on something that's not yet a hazard, don't worry too much. You'll lose one of your available clicks, but you wont be penalised for it unless you run out.