To use the example of a physical file cabinet, if the separate drawers in the file cabinet are represented as the highest level of sub-directories in the file system, then the room the file cabinet is in may be representated as the root directory. That is, other directories may be inside it, but the root directory cannot go in any other directories, at least in that file system.
Note: In popular operating systems such as Microsoft Windows and Unix, you may place files inside the root directory; not only in folders. You may envision this as placing paper files in the room but not in a drawer of a file cabinet.
Unix abstracts the nature of this tree hierarchy entirely, and in Unix the root directory is denoted
/. All filesystem entries, including mounted partitions are "leaves" off this root. However under DOS and Windows, this behavior is different. Each partition has a separate root directory (labeled
C:\\ for a particular partition C) and there is no common root directory above that.
See also: FHS