This just states that a room contains four walls, has two bookshelves and on the bookshelves is any number of items. I cannot tell looking at it if item is a concrete class or not, but we know that both Book and Toy are also of type item.
The interface merely states that anything of type shelf has the ability to addItem and removeItem. I'm not sure if the diagram is incorrect here, but the shelf carries no specific purpose of its own since a wall can only contain a bookshelf and not a shelf. If a wall could contain a shelf, then it should be aggregating the shelf and not the bookshelf.