DIR (short for "directory") lists what is on the disk. It is the MS-DOS ancestor of bash's ls, and it is the command people typed more than any other.

There is a lot in that answer. The three lines at the top name the disk and say which folder is being listed. The two at the bottom count what was found and say how much room is left. In between are the entries themselves.

<DIR> in the middle column means that entry is a folder rather than a file; files show their size in bytes instead. So DOS is a folder, and AUTOEXEC.BAT, CONFIG.SYS and README.TXT are files.