A Content Management System is a tool that helps you manage content.
That sounds very simplistic, but it's still the best definition I've been able to come up with after 20+ years managing content. It's also one of the most useful explanations. Bear with me.
The first question, of course, is: what is content? The term is quite vague. In practice, it can mean a lot of things, from text, to images, video, documents. It usually refers to "unstructured" content (to simplify: it doesn't fit neatly in database column/row/fields, and if you don't work hard at it, it's going to be big blobs with hopefully some metadata).
There are content management systems specialized in different kinds of content (e.g., document management, or asset management for image/video). There are also systems specifically addressing a particular purpose (e.g., a web content management system, which typically manages at least text and images).
The second question is: what does managing content mean? This usually involves managing the "lifecycle" of content. If you do a Google image search for a graph depicting that, you'll find hundreds of attempts. Let's simplify and say it will probably involve create/edit/publish/archive/delete/re-use of content.