坤 is a bound form (b.f.) that appears in classical and formal Chinese, not in everyday speech. It represents the female or receptive principle in the Yijing (I Ching), where it is one of the Eight Trigrams (☷) and one of the Sixty-four Hexagrams (䷁), symbolizing earth, submission, and the feminine. You'll encounter it in compounds like 乾坤 (the universe, lit. heaven and earth) and 坤包 (a woman's handbag), but it rarely stands alone.