Dharana-concentration
Deśabandhaścittasya dhāranā | 3-1
Dharana is fixing one's mind in one place (object/idea).
Mind can be fixed on any chosen object as has mentioned in the first chapter. Try to choose on the gross external object at initial stage, because the tendency of the mind is going outward, then when the meditator is progressing then one can use more abstract idea. In Dharana, one only has one though. It can be done only if one is free from the distraction of the mind. And undistracted mind is possible if only one is firm Yama, Niyama, Asana and Pranayama.
Dhyana-meditation
Tatra pratyayaikatānatā dhyānam | 3-2
A continuous flow of thought is meditation.
When one doesn't shift his/her though away from the object of concentration in a length of time, is meditation. If dharana is drops of water, then dhyana is flow of river. Many concentrations make a meditation.
Frankly said that when one is engross in the transactional activities and there is a lot of worry, grief and others emotional up and down, how can one do concentration or meditation? One might be trained to concentrate, even can be said as a meditator, but when he/she come out from meditation, he/she still feel as the same old person who is facing the same old problems. It does not means that meditation is useless. It does help in to the certain extent for one to develop composure of the mind. Especially by picking up the object of meditation like the calmness of Buddha if one tend to be angry, or the strength of Sri Hanuman if one tend to be weak mentally. With the daily practice, these virtues will create impressions in the mind, in course of time these unwanted tendency will subsides, and even if they come, they can be handled in a shorter time.
Therefore angga (limbs of raja yoga) also called as rungs of raja yoga, because when Yama and Niyama are followed, one is more cheerful and contented, which condition of the mind is possible for meditation. Sage Patanjali prescribes Yama and Niyama for the peace between me and the transactional world, Asanas and Pranayama for the harmonious of muscles, nerves and vital force, and Pratyahara establishes tranquility. We also need to see them as limbs which without any of them, the entire system can not be done.
Samadhi
Tadevārthamātranirbhāsam svarūpaśūnyamiva samādhih | 3-3
The same dhyana, when the object of meditation is devoid of itself, is samadhi.
When there is no devision between the meditator, object of meditation and meditation itself.
The meditator can not know which is the subject, which is the object and which is the process of knowing. He is staying in his true nature/ true Self/ Consciousness, which is not mixing up with the non-Self.