Boxed-off means to separate an area from another area by building a physical wall.
So. Imagine a room. Its a big room, with enough space for a really big lounge, TV.. even a pool table. Its actually TOO big. You would like a small room for storage. So you build a wall on the left hand side of the room and fit a door. The smaller room is "boxed-off" from the rest of the room... because you build a physical separation (the wall), creating two spaces.
Hope this helps.