ELO hell is a perceived rating range in competitive gaming where you keep losing because of bad teammates, not your own skill. Mostly mythical according to game designers. Not concerning, just gamer frustration vocabulary. If your kid uses it constantly, ask them where they first heard it, not to police their speech but to understand which online community they are in. Most slang of this kind moves through TikTok and group chats, so seeing it in your child's vocabulary just means they are plugged into normal teen internet culture.