Usually it means an hazard or an unexpected impediment like water, toys, torn carpeting,clutter and even exposed electrical cords laying in the way of foot traffic.
No trip hazards can be present on carpets. What does that mean?
In some places -- like hospitals -- tripping over can be dangerous. Leaving anything on the floor in a position where it can be tripped on is a "trip hazard" and should be avoided.
Another area applies to old people. I fall over regularly and have to be careful not to leave things lying around. So far I've only broken a couple of ribs but old bones are brittle and it's only a matter of time before I do some serious damage. So trip hazards in and around my home are also to be avoided.