Killua and Gon learn about Nen from Wing, Zushi's teacher. Nen involves using your body's aura while fighting. On their way to register for the fights on the 200th floor, they're stopped by Hisoka. He doesn't think they're ready to fight at a high level. He uses his Nen to block them, and they have 3 hours to learn enough Nen to get past him, or Killua can never fight in Heavens Arena again.