What is a configuration data file and what does it do for the operating system?

Solution
A "configuration data file" would presumably give startup and configuration information to whatever needed it.

That could be the OS, or an application.
No, just want to know for my own information, but can find no answers online or in the text other that it is mentioned as a component of an operating system. I thought the experts here could explain to me what it is and how it helps an operating system work. If not, then thank you all any way.