What are we supposed to learn or understand when we contemplate impermanence?

When the mind contemplates the thoughts that turn it to the Dharma, the first thought is of the precious human body, with its freedoms and resources, with which we can accomplish great things.  

Understanding that this body is impermanent, and meditating upon this, we achieve certainty that we have this great purpose that we need to fulfill, and that we need to embark upon it immediately.

His Holiness, 17th Gyalwang Karmapa

Ogyen Trinley Dorje

From talk at Root Institute, Bodh Gaya, January 2015