The monopod definition, as it pertains to digital photography is: the monopod is a one-legged camera support. A synonym for monood is unipod although that term is rarely used anymore.
Why should you care? Monopods can serve several useful functions for you, some of them similar to tripods, but in a simpler, lighter format.
The Wiki definition of monopod: a single staff or pole used to help support cameras, video cameras, binoculars, rifles or other precision instruments in the field. The monopod allows a still camera to be held steadier, allowing the photographer to take sharp pictures at slower shutter speeds.
That's a pretty simple explanation of what monopods are and what they are used for, but there are other benefits as well.
The primary use of a monopod is to support the weight of a camera to stabilize it and eliminate blur from camera movement. I've also used monopods for a variety of related uses.
Monopods don't automatically come with the ability to tilt or swing your camera to different angles without the use of a rotating head, such as a ball head or pan/tilt head.
You can easily pan from side to side without a special head, but it's awkward to tilt side to side and upward or downward. This is not a big deal at all because ball heads are super affordable for most camera/monopod combinations. Large ball heads are only needed for heavy lens/camera combinations and are more popular with landscape photographers.
A fun and easy to use camera accessory that you can have a blast with and improve your photography. Go ahead. Play around with your monopod. Keep shooting. Keep learning. Keep improving.