A hazard is something that might hurt you or even kill you. In other words, hazards are risks or dangers: things you have to look out for.
For example, a damaged electrical outlet is a hazard. It might electrocute you, or even start a fire.
For another example, some chemicals give off hazardous fumes. Be careful not to breathe them in, or you might get sick. I hope you never have to live near a chemical plant like this one: it could be a hazard to your health.