Samsara

The wheel of life (bhavachakra in sanskrit, or samsara for others) is often found on the exterior walls of the monasteries, on either side of the main entrance. It represents samsara which is a synonim of the word life. The "wheel" also happens to be a synonim of samsara, as it can also means "cycle" or "rotation".
Samsara an Iconographic Concept

Meaning of Bhavachakra

By Samsara (bhavachakra) we are talking of all existences that are conditioned by: ignorance, suffering and the unexplainable flow of time, often represented by Yama holding the wheel of life. Nirvana, on the other hand, represents the world unaffected by negative emotions, which by definition is the nature of true happiness.

The notion of a rotation or cycle, is explained by the fact that humans or beings, as I will call them for the remainder of this article, do not occupy a stable place within Samsara, but depending on their Karma will pass from one type of existence to another.