In brief, that's a study of human health where they try to find answers to our conditions and problems using the basics of the Chinese thought.
All herbs are assigned categories such as cold or warm whereas all conditions have the same categories. Herbs also carry additional categories such as sweet, bitter, sour. Let's say you have a "warm" condition, this would be treated accordingly with a "cold" herb. Also its additional qualities are important when assigning the right herb to treat a specific condition.