That Self is a bank, a boundary, so that these worlds may not be confounded. Day and night do not pass that bank, nor old age, death, and grief; neither good nor evil deeds. All evil-doers turn back from it, for the world of Brahman is free from all evil.

In plain words: The Self is like a riverbank or a dam that keeps the worlds in order. Day and night, old age, death, sadness, and even good and bad deeds cannot cross over it. Evil cannot reach it, because the world of Brahman is free from all evil.