"We are the 99%!" is a phrase that was coined by someone in and during the "Occupy Wallstreet" movement. It spread by word of mouth and through the media, including a parody by South Park. It refers to living in poverty or "being part of the common man", as most of the wealth is in the 1%. It is used as a political slogan, internet meme, and in a comedic way.
"Our whole family used to have jobs. Now we don't have any. We are the 99%!."
"Fuck the man! The system is unfair. We are the 99%!"
"I stole this fifth, you know why? Because I am the 99%!"