The fifth limb of Ashtanga yoga is pratyahara. Each limb builds on the next and each relates to having a healthy and fulfilling life.

Pratyahara means “drawing back” or “retreat”. The word ahara means “nourishment”. Pratyahara translates as the practice of turning the mind to introspection by voluntarily shutting out distractions provided by the senses. In other words, non-attachment to the external world and the chatter in our minds.

Most often, we follow our senses. The senses lead us to crave things. When one meditates, pratyahara occurs because we are so absorbed in our meditation that the senses follow the focused mind instead of the mind following the senses.

We are human and thoughts are going to flow. In pratyhara, you don’t cut out senses altogether but you cut out the cravings and create non-attachment to them. For example, you eat because you need food to live, not because you crave to fill a void in your life. When a person lets external situations and sensations influence him, there is an imbalance and inner peace is not reached.

Patanjali says attachment is the root of human uneasiness. If you truly practice non-attachment and free yourself from distractions, your meditation will go deeper than relaxation.

Resources:
http://www.swamij.com/yoga-sutras-25455.htm

Source : http://www.examiner.com/x-8997-Seattle-Yoga-Examiner~y2009m7d19-The-yoga-sutras-of-Patanjali-pratyahara