To expound means to explain something in detail, often at length. When a teacher expounds on photosynthesis, she carefully explains how chlorophyll captures light energy, how water and carbon dioxide combine, and what happens at each step of the process, going well beyond a simple statement like “plants make food from sunlight.”

The word suggests a thorough, almost generous explanation. A coach might expound on proper batting technique, breaking down grip, stance, swing path, and follow-through. A friend who loves astronomy might expound on black holes, explaining what they are, how they form, why light can't escape them, and what scientists have discovered about them.

Sometimes people expound when a shorter answer would do just fine. If someone asks what time the movie starts and you expound on the theater's history, the director's other films, and your opinions about cinema, you've probably said more than necessary.

Expound often appears with the word “on” or “upon”: a scientist expounds on her research findings, or a historian expounds upon the causes of a particular war.