There is some pretty good info about CSS for free on the Web.
In simple(-minded) terms, I think the idea is that you define all the standards and specificatons (fonts, sizes, etc., for each situation) in one place, giving a name to each combination of specs. That CSS file is included in all of your html or php files, so you then invoke the set of specs you want in a particular instance by the name given to them in your CSS file.
The result is that you get a consistent look across your entire website. If you can get a look at the CSS files for some websites that you like, you'll get an idea of how to build and use them.
I am reminded of the time many years ago when I was doing some consulting that involved coding for an IBM mainframe computer. There was a header file that was always placed at the start of each program for that beast. When I asked an experienced programmer how to write that header file, he told me, "Don't try to write your own. Just borrow a copy from someone else's program. Nobady really knows how to write the header file or who did it originally anyway."
Good luck!
Eric