氪 (kè) means krypton, but in gaming slang it's a homophone-ish play on 'spending real money' — the Chinese
课金 (kèjīn, paying real money in mobile games) was abbreviated and rewritten as
氪金 (kèjīn). Tier ladder:
白嫖 (báipiáo, free-to-play) →
微氪 (light spend) →
中氪 (medium) →
大氪 /
重氪 (whale) →
氪佬 (krypton overlord, big whale).