Dhāraṇā is binding the mind to a place
Dhāraṇā is binding the mind to a place. It is binding the mind, as a purely mental process, to the navel circle, the heart lotus, the light in the head, the tip of the nose, the tip of the tongue, and other such locations; and to external objects.
Dhāraṇā (concentration) is binding the mind to one place. Binding to one place means binding it there, and it is the mind that is to be bound.
The commentator gives details, binding to the navel circle all the vital currents meet there in the form of a circle, so it is called the circle of the navel. On the form of the heart lotus, the light in the head. The door of the nāḍī nerve-channel of the head is radiant, and so it is called a light. To the tip of the nose, the tip of the tongue, and other such locations, and to external objects, such as the moon. To these the mind is bound.
The mental process (vṛtti) of that mind, held in those places without being dispersed, is called dhāraṇā, as a purely mental process: it functions simply as the idea of that place without any disturbance (vikṣepa).