Those opposites are in a constant state of flux, there is always a tension between the opposites. Day will be night, night will be day. This constant flux can be seen when you step into a river, it remains a river but the river itself constantly changes. It's waters are always new.
He believed that most people sleep-walk through life, never truly being away of the balance needed and the constant state of flux we live in.
Book The philosophy book
Internet encyclopedia of philosophy.
Stanford Encyclopedia of Philosophy.