Each time a visitor opens your site, the browser sends a request to the hosting server, which executes it and provides the necessary information as a response. A simple HTML website uses minimal system resources for the reason that it is static, but database-driven platforms are more demanding and use much more processing time. Each webpage that's served creates 2 types of load - CPU load, that depends on the time the hosting server spends executing a particular script; and MySQL load, that depends on the number of database queries produced by the script while the user browses the website. Larger load will be produced if loads of people look through a given website simultaneously or if a considerable amount of database calls are made simultaneously. Two good examples are a discussion board with tens of thousands of users or an online store where a customer enters a term inside a search box and a large number of items are searched. Having detailed stats about the load which your site generates will help you optimize the content or see if it's time for you to switch to a more powerful type of hosting service, if the website is simply getting very popular.