Here's a small explanation from me. Although technically not very accurate, it could explain you the concept.
A browser can understand only HTML. A person who updates the content (e.g. site owner's employee or site admin) doesn't understand HTML. So an interpreter is required between the two. CMS is one such interpreter.
A simple interface is provided to update the content (and other settings of the site). The site admin logs in using a username and password, simply clicks a few times
to reach at the right spot and modifies the content. It could be as simple as logging in a hotmail/yahoo account and composing a mail. Once the data is saved, it gets saved in the database which will make it appear at the right place in the right format.
A typical CMS like (Wordpress, Joomla, phpNuke, etc.) is written using PHP and uses a MySQL database.
At times, even if a person has knowledge of HTML, PHP, etc. it is better to use a CMS. For example, in a blog there are various posts which are classified as per category. They can even be in multiple categories. They are also classified date wise. If someone makes a post without CMS, then he will have to manually edit multiple pages. This is a mechanical task prone to human errors. So it is better if a CMS (e.g. WordPress) does the job which is more quicker and more accurate.
Hope I have made it clear.