To completely understand this, we need a little comparison. I'd choose the one from the other side of the DC universe, and pick Marvel's own Deadpool. Deadpool has one of the strongest fourth wall breaks. Although Wade Wilson didn't have this power, Deadpool seemed to have this to the max. A fourth wall break is somehow speaking directly to the audience through an imaginary wall, preferably a film or through a picture, or it could be knowing that you are in an illustrated story.
Breaking The Fourth Wall Like Deadpool
Several characters in have the fourth wall break, and it's not just in the comic book universe. There are those who have it in Phineas and Ferb, up to even Doctor Who.
But there is one particularly, I think, has the most powerful 4th wall break. He is the Clown Prince of Crime, Joker. Many have already seen Joker as knowing that he is a comic book character, although he doesn't really do it that much.
His FWB isn't just onto him, or his life. His Fourth Wall break is so strong, he knows every life's existence and their past. He can see the horrendous things done to others, and what they've done to get even. He's seen things that all the way from the very beginning, of every life. His FWB doesn't stop there. He may have already seen how the universe shaped up. He may have seen the Brothers going into battle themselves, having created the multi-verse. He maybe seeing the other heroes from the other comic book companies.
He Knows Spider-Man
With this picture at hand, he knows who Spider-Man is, even though this is the first time they've actually met. This actually may be proof why Joker has so many stories about his life that don't actually add up on how he became the Joker. It's because he knows so many lives, his mind gets confused on whose life his really is. He even once said that if he had a past, he wanted it coming to him as "multiple choice".
He maybe referring to people who had a horrifying past but is, now, having a bright and joyous life or the ones who always had the bright future ahead of them yet chose to have the terrifying lives.