Demystifying a DOS program

I am trying to figure out the quirks of an old DOS program, written by someone in my company who has since left. There are no manuals for it, and no one currently here has really used it much. I am trying to figure it out by trial and error, but there are some things I can’t figure. Is there a way I can see what it does or read the code? I tried “edit” and “write” commands, but all I see is a bunch of gibberish. Can someone help me?