When a visitor opens your website, the browser sends a request to the hosting server, which in turn executes it and supplies the necessary information as a response. A simple HTML Internet site uses minimal system resources as it's static, but database-driven platforms are more requiring and use more processing time. Each page that's served creates two types of load - CPU load, which depends on the time the server spends executing a certain script; and MySQL load, that depends on the number of database queries created by the script while the user browses the site. Greater load will be generated if a lot of people look through a given Internet site simultaneously or if a lot of database calls are made concurrently. Two good examples are a discussion board with tens of thousands of users or an online store in which a client enters a term within a search box and tens of thousands of items are searched. Having in depth statistics about the load which your website generates will allow you to improve the content or see if it is time to switch to a more powerful kind of hosting service, if the Internet site is simply getting quite popular.
