When a character has one foot planted on an object on the ground.
Do not use this tag if the character is stepping on a person. Use Stepped On instead. If it's on the clothes they are wearing, use Stepping On Clothes. If stepping on the frame of the image, use Stepping On Viewer. If they character is stepping forcefully, use Stomping. If the object is large enough that the character has both feet on the object, tag with Uneven Footing instead, like On Stairs and vehicles.
If stepping on the object triggers something to happen, like it's a Booby Trap or a device, tag with Stepping On Trigger.