What is a Danbuoy? Danbuoys at a basic level are a cylindrical shaped buoy with a weighted pole underneath and a tall thin fibreglass pole extending out of the top with a flag on the top. You get variations on this theme depending on the level of sailing you are doing, but they all share the same characteristics
Offshore Danbuoys are based around the same principal but are generally much bigger with extendable poles on the top and often fitted with a light as well to cope with larger swells and sea states further away from land, as well as being visible at night.
You also get inflatable Danbuoys - like the Jonbuoy Danbuoy - which do exactly the same thing, but inflate when they hit the water. They're often desired as they are generally the easiest to mount and store on a boat.
Why would you use a Danbuoy? A Danbuoy is used or deployed in a man overboard situation. Dropped off the back of the boat it marks the approximate location the boat was when, or if, a crew member goes overboard. Giving you a spot to aim for and search for the casualty as boats, yachts in particular, can't just stop on the spot if someone goes overboard.