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