they're the text parts of the website, the parts that make up articles. they are put into existence by using the "p" command, as demonstrated in this gif.
this style extension applies to headers as well. this is the main way for your sites to pop out, and look cooler than other sites.
They can be used to make a news site look more like a newspaper, to show the breaking news immediately, for example..
It can also be used to catch someone's attention immediately, to get something to someone's eyes as soon as possible. for example...
And they're super easy to implement into a website, if you have no real experience with CSS. They are implemented by adding a "style" to a paragraph or header block, as demonstrated here.