Dharana is to focus on a specific object.
Concentration is a mental skill. There are two ways in which the mind can concentrate on a single object using the will and ability to steadily focus: consistent energy expenditure and stress overload OR by ensuring that the object is pleasant and interesting. The latter condition means that the concentration comes naturally without effort or tension.
In Yoga almost all the exercises of Concentration are pleasant and stimulating for both the senses and the mind. While in meditation position, i.e. comfortably seated with the spine straight, we close the eyes and practice Concentration.
Mental activity is reduced and the body is relaxed until one reaches a state of profund stillness and peace.
Then we are concentrated and prepared to practice Meditation.