WHAT IS IT...... A software application for you to manage and administer the content of your website without having to learn to code.
WHY I NEED IT..... If your website or app content is updated or changes regularly, and you are not a trained IT administrator, It'll come in handy. It can help you manage SEO, feeds, forums, blogs and even user management.
There are hundreds of CMS out there. Some notable and popular ones are Wordpress, Joomla and Drupal. Picking the right CMS for your app or website takes into account things like type/features, scalability, hosting, cost, support as well as update frequency (security).
It is easy to focus on design and function when developing your app or site. Maintenance, management and administration are just as important, so make sure you have the conversation with your developer.
If you'll like to do a side by side comparison of various CMS, try cmsmatrix.org.33