Almost everyone has experienced the fact that when one starts concentrating his mind on any immediate object or an idea, the mind starts wandering. It is very difficult to keep the mind busy with a single thought. The ancient sages too encountered the same problems. Arjuna, in Gita had mentioned that controlling the mind is an impossible thing on earth. Hence, he was advised by Krishna that though mind control is difficult, it can be made silent and steady by regular practice of vairagya and abhyasa. However, he has warned that yoga is very difficult for people whose minds are not steady and controlled. Pantanjali, in Yoga Sutra has emphasized these two qualities to achieve mind control. Hence ,these are the very qualities that make the essence of yoga.
Mind is like a disturbed pond with many impurities. First you need to stop the inflow of fresh impurities and then remove the existing impurities to clean the mind. Abhyasa is one of the practices for purifying the mind. Dhyana is one of the sub practices of abhyasa. This is a stage that a person reaches after practicing concentration for some time. At the start of dhyana, the mind is steadier and only a single thought about an object arises in the mind. Now it is safe to say that the state of dhyana is reached. Here, the mind becomes very stable like the flame of a lamp in a calm atmosphere and its contact with the object of experience becomes intense and complete.
There are two varieties of dhyana called sagunadhyana and nirgunadhyana. In the first dhyana, the stillness of mind is associated with an object of experience which can be experienced through the sense organs. The second one is completely mental. It implies complete absorption of mind into itself. Here, the mind is not associated with any external object. It becomes completely still in this state. This mind is supposed to remain still, silent and sensitive such that it can understand any past, present and future event that might have happened anywhere in the universe.