If something's closed, it's shut or sealed. When your brother's bedroom door is closed, you'd better knock; if the library's closed, you'll have to come back tomorrow to check out some books.
Things that are closed aren't open. People sleep with their eyes closed, and your neighborhood bookstore might be closed on Sundays. Things can be figuratively closed too: if you have a closed mind, you're not willing to consider new ideas or perspectives. The word closed comes from the Old French verb clore, "to shut" or "to cut off from," with its Latin root claudere, "to block up or make inaccessible."