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.
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.