If someone is landed, it means they own property or acreage, and they probably inherited it. In 17th and 18th century England, owners of country estates were known as the landed gentry.
This adjective almost always comes before "gentry" or "aristocracy," referring to an entire category of wealthy people. It's a bit old-fashioned, but you can still call use it for upper-class land owners. Your cousin, lucky enough to have the family estate handed down to him, is a member of the landed gentry. Landed can also describe the land itself, when someone inherits it: "The landed property was about fifty acres."