Indefinite Pronouns in Korean grammar are used to refer to non-specific persons, things, or quantities. They are essential in expressing ideas and concepts without specifying a particular subject or object. In Korean, indefinite pronouns are often formed with the ending -ㄴ가/누가 (nuga) for people, -마다 (mada) for every/all, and -든/아무 (deun/amu) for any. Understanding and mastering these pronouns are crucial to achieve fluency in the Korean language.