If you run a website, sometimes you need to make changes to the (X)HTML/CSS to fix or improve your website's styling.
Sometimes these changes can be pretty drastic in terms of layout changes. Browsers commonly cache CSS files. If a visitor's browser has cached the old version of your CSS file, they may see a very ugly version of your site, applying the old stylesheet to the new markup.
The principle is simple. just use a query string on your CSS call - e.g.
<link href="/css/your.css?date" type="text/css" rel="stylesheet" />
Every time you update the CSS file, simply change the query string to that date. This forces any browser to download the new file, which it will cache until the next time you update your query string, or until something else makes the cache expire.