排位 is short for
排位赛 (páiwèisài, ranked match) — competitive matchmaking in games like
王者荣耀 (Wángzhě Róngyào, Honor of Kings) and
英雄联盟 (Yīngxióng Liánméng, League of Legends). Contrast with
匹配 (pǐpèi, casual / normal mode). Also a noun for one's rank itself:
我的排位 = 'my rank'.