Dharana, concentration, is centring the mind on one single thought. Vedantins try to fix the mind on the Atman. This is their dharana. Hatha Yogins and Raja Yogins concentrate their mind on the six chakras, energy centres. Bhaktas concentrate on their Ishta Devta, tutelary deity. Concentration is a great necessity for all aspirants. During concentration, the various rays of the mind are collected and focused on the object of concentration. The whole energy of the mind is concentrated on that one idea. The senses become still. They do not function. When there is deep concentration, there is no consciousness of the body and surroundings.
When you study a book with profound interest, you do not hear if a man shouts and calls you by your name…. You do not smell the sweet fragrance of flowers that are placed on the table by your side. This is concentration or one-pointedness of mind. You must have such a deep concentration when you think of God or the Atman.
There is great concentration when you play cards or chess, but the mind is not filled with pure and divine thoughts…. You will have to fill up the mind with sublime, spiritual thoughts. Then only will the mind be expurgated of all worldly thoughts.