A monopod is a device for camera stabilization that has a single leg. Monopods are similar to tripods, albeit with 1/3 the leg in that they usually use a quick release plate to affix the camera to a head that is screwed into the leg. Monopods can be advantageous in that they are often allowed in places that tripods aren’t but stabilization is required for crisp shots.
Sometimes monopods are used to support very long, heavy lenses.
They can also be used in videography for some effects, like a faux dolly-in.