天荒地老 is a four-character idiom (成语) that literally means 'until the sky decays and the earth grows old' — a poetic way of saying 'forever, until the end of time'. It expresses an eternal, unchanging commitment, often used in romantic contexts (e.g., vows of love) but also in literary or dramatic descriptions of lasting friendship or loyalty. It is more formal and literary than everyday words like 永远.