It’s used to be a file that was executed by default when starting up older computers, up until Windows 98 I think (?). It was a simple file in which you could insert commands to personalize and/or optimize your system, back in the days.
It was very powefull if you knew what you were doing, but could be very harmful if you had no idea. Like you could litteraly erase your hard drive by simply adding "del . /q" to the file if I remember correctly.