To be honest, I never really got a good description for myself, just through experience I know when to use one.
A good answer will not need to include the definition of "block element" or any other technical jargon, although you can assume some basic knowledge of some HTML tags
For some reason the purpose or logic around the
<div> </div> tags, made sense to me looking at cloth clips, and the reason it is because:
( <div> </div> )
<div>alone can not work.