Pratyahara is the 5th step of the Ashtanga yoga and is the first step of the meditation process. There are 3 steps in the meditation process: the withdrawal of the senses (Pratyahara), concentration (Dharan) and meditation (Dhyana).
Patanjali defines meditation as follows: "If the mind is capable transcend the awareness of smell, sound, feeling, form and taste and at the same time let the consciousness function focussed on one point". The goal of Pratyahara is to internalize the senses and eliminate the impressions from the communications between the senses and the brain. From birth we are solely conditioned to be aware of the outside world. The goal of Pratyahara is the internalize our awareness.
Pratyahara prepares the next two steps: Dharan (concentration on an object) and Dhyana (meditation). In order to be able to concentrate or the meditate we must remove the obstacle of the disturbed mind. The mind constantly receives impulses and information from the outside world through the senses, these impulses are food for the mind. Our whole being is mostly focused on the outside experiences and it is very difficult for us to internalize the mind into our awareness. This is a difficult obstacle to overcome.
Pratyahara is meant to disconnect the awareness from the senses and to trigger the internalization of our mind. A lot of people come to the conclusion that a too big an effort to try to forget the outside world leads to a heightened awareness of the external perception. The initiation of Pratyahara depends an regular practice and your state of mind.
A peaceful mind will make Pratyahara easier, for a disturbed mind it can take months before the disconnection with the outside world is obtained. Without controlling Pratyahara it is impossible to start Dharana or Dhyana. Our mind maintains a constant contact between the senses and the intellect in the present. At the same time it is occupied with the memories of the past and fantasies about the future. The goal of Pratyahara is to eliminate in the present the constant contact of the senses and the intellect.
We can help ourselves to eliminate the external disturbances by practicing the prior steps of the Ashtanga yoga. The Yamas help us to eliminate the disturbances caused by other people. The Niyamas help us to undo the disturbing factors of body and mind and de asanas help us to lessen the disturbances that are caused by the limits of our body, while Pranayama helps to eliminate the negative consequences of bad breathing.