COMMAND.COM is a DOS Executable, that contains the DOS Command Processor. This file is needed for DOS to understand its 'internal' commands. So this file that is included with DOS, or an alternative, is needed as well as the 2 hidden files, IO.SYS and MSDOS.SYS (called IBMBIO.COM and IBMDOS.COM on IBM PC-DOS, but otherwise identical).
It maybe run a second time, then EXIT typed to return to the first one.
Some consider these three files to be DOS, while others include all on the discs.