Think of a Content Management System (CMS) as a word processor for your website. It provides you with a What You See Is What You Get (WYSIWYG) interface so that you may add, edit or remove your content as needed.
WordPress, for example, is one such CMS. WordPress has many features to help the small business owner manage their own content including page details, blog posts and image galleries.
A big advantage of having a CMS is that you, as a small business owner, can manage your own content instead of having to pay someone to do it for you. Over time, this could save you a ton money that you can reinvest in your business.