What is Isekai?

Isekai (異世界, "another world") sends a present-day person into a world of swords, magic, or game mechanics — sometimes summoned as a hero, sometimes reborn there, sometimes just yanked through a portal.

The hook is the fish-out-of-water power fantasy: modern knowledge (or a cheat skill) makes the newcomer special in a world that runs on different rules. It's a Japanese term, but the trope is everywhere in manhwa and manhua too.