Dharana in Yoga is about mastering single-pointed concentration, and the sixth step of the Yogic practice as described in Patanjali’s Yoga Sutras.
It’s about training the mind to stay focused and concentrated on one object only, while avoiding all other thoughts and sensations. Dharana can be translated as “holding firm”, “holding steady”, “concentration”, or “single focus”.
The object one chooses to concentrate on can be literally anything: an internal thought or idea, an external object around you, or one in the outside world, or perhaps a Mantra.
Practicing Dharana is seen as the preliminary step to be able to perform Dhyana (deep meditation), which is the subsequent step (or limb) in Patanjali’s Yoga system.
Mind that in the Dharana stage there’s still a separation between subject (the one who does Yoga and concentrates) and object (the thing that is focused or concentrated on).
The goal of the subsequent steps (Dhyana and Samadhi) is the complete suspension of a subject-object relationship, that is, to become one with the object of meditation. Unlike Dharana, in Dhyana and Samadhi the object one meditates on is typically God, the Absolute Self, or the Divine.