The monkey mind is an analogy in yoga that describes a mind that will jump from thought to thought as quickly as a monkey jumps from tree to tree. The monkey mind is never focused on the present and is in a constant state of distraction. Early commentary on the Yoga Sutras discuss how the mind has five basic patterns or states;
A restless mind is one that cannot remain focused for any length of time, nor stay fixed in any singular moment (i.e monkey mind).
A stupefied mind is one that obsesses, ruminates, and dwells over problems such as conflict heartbreak or disappointment.
A distracted mind is one that can concentrate for short periods of time, but continues to revert back to distraction.
A one-pointed mind can rest attention on any object or subject that it chooses to contemplate (this could be pranayama / the breath, a mantra, or another mental or physical task).
A completely restrained mind has no fluctuations and no object separate from yourself to hold onto - leaving only your consciousness as your experience.
The practice of yoga can help train the mind and harness the power of attention. When you practice asana in combination with pranayama, you become entirely focused on the movement of the body and the breath - it can control that monkey mind.