This is more of a meta-setting that should probably be a tag, but it originated in “The Gamer” manhwa and became a widespread fanfiction tool.
Basically the protagonist - or more people, or rarely the entire population of the world - is like an RPG character with stats that affect them directly.
Various other things may or may not be present, such as leveling, quests and dungeons, skills/abilities, “Gamer Body” and “Gamer Mind” etc.