When hiking you can expect that the trail is not smooth like a sidewalk or even like a footpath in a park. It can be full of rocks (remember Pennsylvania?) or roots. Vermont has been full of roots. When the dirt around the root has been eroded away so that it looks like an arch in the middle of the trail–that is a tripper. Or when the root has been broken off and is sticking straight up–that is a tripper. Sometimes the root hasn’t been eroded away but you can’t see it because of the overgrown weeds, ivies, little trees, grasses etc–that it is a tripper.
Vermont has been full of trippers, so many in fact that we’ve ceased to warn each other about them, or it would be the only words we would speak all day long.