The previous 5 limbs are concerned with the preparation for meditation, without preparation, the mind is unable to concentrate; trying to meditate with no preparation will lead to an inner battle; you cannot force the mind to concentrate. However, with preparation, concentration comes more easily. These last three limbs bring about total mental relaxation through a process of mental self discipline.
Dharana means controlling the internal mind through concentration, (holding or fixing the attention of mind onto one object or place). The ability to concentrate is the path to higher qualities of mind, but this is not easy and requires sustained and regular practise.