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 the one of the practices for purifying the mind. Dhyana is one of the sub practices of abhyasa. This is a stage that the person reaches after practicing concentration for some time. At the start of dhyana, the mind is steadier and only a single thought about the 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 the calm atmosphere and its contact with the object of experience becomes intense and complete.
There are two varieties of dhyana called sagunadhyana or 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 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.