Your web browser sometimes stores some basic assets (images, layout, etc) in the cache to speed up load times and reduce bandwidth usage. This is a good thing for day to day browsing, but can cause issues if you are working on your site or trying to change something. Clearing the cache forces a fresh copy of the site to be downloaded, allowing you to see recent changes.
To know more about how caching works, please see this Wikipedia article.
Each browser is independent and has their own steps for clearing the cache. Please click on the link for the browser you use for directions on how to clear your cache.