Browser caching works by marking certain pages, or parts of pages, as being needed to be updated at different intervals. Your logo on your website, for instance, is unlikely to change from day to day. By caching this logo image, we can tell the user's browser to only download this image once a week. Every visit that user makes within a week would not require another download of the logo image.

By the web server telling the browser to store these files and not download them when you come back saves your users time and your web server bandwidth.