|Whether the outer perimeter of something is fenced.|
Once upon a time there were no barrier tags, because nobody had invented them yet. In that time the only way to map a fence in Openstreetmap was with fenced=yes. Almost all fences mapped since the invention of barrier=fence are (re)mapped using that new tag.
The reason this page still exists is, to let people finding objects tagged with fenced=yes know what it means.
So for general information on how to map fences and associated features please see barriers.
This tag is not rendered by the main Openstreetmap renderers.