A monopod, also called a unipod, is a pole used to help support cameras, video cameras (or, less frequently, binoculars). It allows a still camera to be held steadier, allowing the photographer to take sharp pictures at slower shutter speeds, and/or with longer focal length lenses. In the case of video, it reduces camera shake and therefore most of the resulting small random movements. When used by itself, it eliminates camera shake in the vertical plane. When used in combination with leaning against a large object, a bipod is formed; this can also eliminate horizontal motion.
Monopods are usually made to fold easily or quickly, or "telescope," when not in use, allowing them to be transported and stored more easily.
Unlike a tripod, monopods cannot support a camera independently. In the case of still cameras this limits the shutter speed that can be used. They still allow lower shutter speeds than hand holding, and are easier to carry and use than a tripod.
Many monopods can also be used as a "chestpod," or "beltpod," meaning that the foot of the monopod (sometimes with a special adapter) can rest on the belt, waist, or chest, of the photographer. The result is that the camera is held more steadily than by hand alone (though not as steadily as when the foot is planted on the ground), and the camera/monopod is completely mobile, travelling with the photographer's movements. This is similar to a finnstick.
In terms of mobility vs. stability, generally if stability increases, mobility decreases. From most stable/least mobile to least stable/most mobile: tripod/tablepod/resting on surface of some sort, monopod, chestpod, handheld.