tatrekaagram manaha kritvaa yatachittendriyakriyaha |
upavishyaasane yunjyaadyogamaatmavishuddhaye || 12 ||

Seated on that seat, making the mind single pointed, having subdued the activities of the mind and senses, engage in the yoga for purification of the self.
tatra : that
ekaagram : single-pointed
manaha : mind
kritvaa : making
yatachittendriyakriyaha : subduing activities of mind and senses
upavishya : seated
aasane : seat
yunjyaat : engage
yogam : yoga
aatma : self
vishuddhaye : purification
In this shloka, Shri Krishna provides a comprehensive introduction to the process of meditation, and also points out the goal of meditation. He says that the goal of meditation is to purify the intellect. The process to do that is by sitting down, controlling the mind and senses, and focusing the mind, making it single pointed.
First let us examine the goal of meditation which is the purification of the intellect. The shloka says “aatma-vishuddhaye” meaning “self-purification”. Note that the word self means the intellect here. But what does purification of the intellect mean exactly? To understand that, we need to take a step back and examine where meditation is placed with respect to the topics in the Gita. Meditation is the culmination of the entire curriculum of the Gita which aims at removing the three main defects of our personality: mala, vikshepa and aavarana.
In the third chapter of the Gita, we encountered karmayoga. It enables us to purify the first impurity of the personality known as mala or dirt. Mala causes the mind to rush out into the world of sense objects due to a sense of finitude. In other words, it causes us to act in the world. As we have seen earlier, we use karmayoga to turn this defect against itself, just like a thorn removes another thorn. Next, devotion or bhakti is used to remove the second defect known as vikshepa, which the tendency of the mind to get distracted. Chapters 7 to 12 examine bhakti in great detail.
After dealing with mala and vikshepa, the Gita addresses the last defect known as aavarana or covering. This covering prevents us from fully getting established in the knowledge of the eternal essence. Having gone through shravanam (reading and listening to knowledge) and mananam (removal of doubts), meditation helps us achieve the last step which is nidhidhyaasa or internalization of this knowledge.
So the notion that “I am not this body, I am the eternal essence” needs to be fully internalized so that it becomes second nature to us. This is the goal of meditation.
The process to achieve this goal is also addressed in this shloka. Shri Krishna asks us to sit down on that seat that was described in the previous shloka. He then asks us to make the mind single-pointed or focused. But this is easier said than done. Whenever we close our eyes and sit in meditation, the thoughts of the world rush in.
Many techniques are given in other literature in order to make this happen, including concentrating attention on a point on the wall, on a flame and so on. But the prerequisite to all of this is that we have to subdue the mind and the senses.
Consider the example of the person that wants to remain seated inside a room. Either he will have thoughts of objects outside the room, or someone from the outside will want him to come out of the room. Similarly, when we sit for meditation, either the mind will generate thoughts of the world, or the sense organs will pick up a scent, a touch and so on that will drag the mind out. Therefore, we have to control the mind and senses before attempting meditation.
There also are some secondary benefits to meditation. If we meditate on a regular basis, we will never be reactive to situations. No matter what kind of situation comes up in front of us, we will always take time to pause and then decide. This is because the mind has already been trained to slow down the thinking process. We will then not lose our balance and not always work in a reactionary mode. We will take a good look at situation and slowly approach it
meditative person will have this advantage.
Next, Shri Krishna delves into the specifics of meditation.