What does it mean to be a brony?

There is a difference between being a ファン and being a brony(or pegasister). A ファン just watches the 表示する and enjoys it, and that's fine. A brony is touched によって the 表示する on another level, and therefore, joins the brony community. When you're a brony, friendship truly is magic. あなた help your フレンズ when they're in need, and stand up against those who try to put あなた down. あなた can have fun talking about MLP:FIM, 書く fanfics, drawing pictures, RPing, and so much more.
That's what being a brony means to me. :)

Basically what DragonAura said.
Bronies/Pegasisters are really in touch with the show, as they know a ton about MLP and make a bunch of fanart of all sorts, like drawings, music, and fanfics. They talk about it on both internet and real life. Go to conventions like Bronycon and Everfree. Besides, it's not just those. It's believing the true meaning of friendship and happiness. Making フレンズ near または far.