If something is a hazard, it is a potential source of danger. Balloons are fun, but they're a hazard for little kids who might put them in their mouths.
If you play miniature golf, you have to watch for hazards on the course — obstacles like the sand trap beside the dinosaur or the water hazard just past the windmill. As a verb, hazard means to take a risk, especially for the chance of a good return. You might hazard your chances at the roulette table or hazard a guess — that is, risk making a guess when you aren't certain.