Display: Block (stoopid noob question)

Okay, being self-taught on HTML and CSS, I know I’ve missed all sorts of things that are basic. So, here goes:

What is the function of the display: block property? I’ve never used it, but recently, while studying other people’s markup and code, I see that property in some cases. A visit to W3C gave me a technical definition, but not a practical example that shows me why I need to use it.

An explanation, please? thanks!!