To be understanding is to be sympathetic to someone’s woes. Understanding a concept means you get it. Your understanding might be that your mother will always drive you to school if you miss the bus.
The sum of your knowledge of a certain topic, is your understanding of it. This can change, or deepen as you learn more. But being an understanding person doesn't take a lot of studying — it takes opening your heart to appreciate what someone else feels or experiences. If someone says to you, "I thought we had an understanding," you must have done something unexpected. Here, understanding means "an agreement."