CSS stands for cascading style sheets.
There are three different ways to apply CSS to an HTML document that you'll commonly come across:
External stylesheets let you add the same style features across several html files in the same directory, but I can't get this to work at the moment. It looks correct... I suspect Neocities is just quirky sometimes? I had issues earlier too with the editing i did not loading. edit: I was correct. It just needed time to load the changes in.
Inline styles is the least effective implimentation of CSS across several webpages.
Internal stylesheets are sort of a happy inbetween. Useful when called for.