If you do a search for CSS, you will find many many articles on style sheets.
But a basic explanation....
Instead of putting formatting (colour,fonts, etc) in your html page, you are instead assigning a
class or
id to each tag as an attribute.
Example:
<p class="paragraph1"></p>
instead of...
<p font face="verdana" size="2" color="#ffffff"></p>
So, you have taken all the excess code out of your html page, & have it able to be called on for use (even with other tags, and other pages) in another file.
At the top of your page you are calling a style sheet to be loaded. When the page loads, it calls from the
.css file (the stylesheet) the
class or
id to apply into that particular area of the html page.
Hope this helps .