Your web browser stores a copy of every site you visit on your computer in special folders it calls its cache. This saves a copy of every HTML page, graphic, sound, and other static content that your page needs. This cache is made so that subsequent requests for the same page will load faster, as the images and media are already downloaded to your computer.
It's important to clear your cache at times, especially when your hard at work on your site and you want to see what the latest version looks like. If you don't clear the cache, you may be seeing what your browser displayed last time it actually downloaded from the server. By clearing the cache, you tell your browser, “Forget what you have downloaded, and go download it again”. This causes you to get the most recent update of what is on the server.