The inverted pyramid refers to an almost required format in print journalism, whereby everything that is important must be revealed at the beginning of an article.
If you were to, for example, take a pyramid and flip it over on its point, all of the weight of the pyramid would be at the top, with a tiny little insignificant point at the bottom. This is how we like for news articles to be done.
So, if you were to be writing a news article, you would begin by telling your reader everything they could ever want to know almost immediately, making sure that by the end of your third paragraph your reader can stop reading, feeling fully informed, and go back to the important business of watching reality television.
This bedrock of journalism, as you can imagine is very popular with editors, who have places to go and articles to read and want everything in the article to be self-evident at first glance. It is not so popular with writers and reporters who fancy themselves to be articulate storytellers.