A Content Management System is a tool for giving you control over your own website.
A good CMS means that your site can evolve to meet the demands of your customers without you having to keep paying someone to make changes to it.
How does it work ?
Think of the CMS as the back of the shop, and the website as the front.
The CMS is accessible to the staff, but not the customers, through what is effectively a locked door : a staff login page on your site.
Once on the other side, they can view the whole site in a different format : each page being divided into a number of boxes, or fields, which can be edited through your web-browser.