Basically a variation on skanking. Which was a dance you did while listening to ska or punk rock music. Skanking is usually done in a circle pit. 2-stepping is done usually during a breakdown of some hella brutal metal music. You don't go around the pit or anything. You usually just stay in once place and 2-step that shit. Hardcore dancing is sometimes referred to as "hardcore 2-stepping" or "slam dancing".
"Fuck skanking, I wanna 2-step!", "Let's 2-step at the show tonight."
FYI: I have no problem with skanking. I'd rather skank to Reel Big Fish or Rancid than hardcore dance to some crazy hardcore music.