Pranayama, or the control of Prana/inhalation and exhalation is precursor to Meditation, God realization and Salvation/Moksha.
PRANAYAMA AS PER SAGE PATANJALI
"Tasmin sati svasaprasvasayor-gativicchedah pranayamah"? Yoga-sutras of Patanjali. So Sage Patanjali, in his Yoga-sutras defines Pranayama as "The process of regulation of breath or control of the Prana, is the stoppage of inhalation and exhalation, which occurs after attaining steadiness of posture or seat."
The 'Svasa' means inhalation and 'Prasvasa' means expiration of breath. You can get into the practice of Pranayama after you have gained steadiness and comfort in your Asana (the posture of seat).
After you have been able to sit comfortably for 3 hours continuously in one Asana, that means you have gained mastery over that particular Asana. But even if you are able to sit from 15 minutes to one hour even, you can continue the practice of Pranayama. You have to keep in mind that it is the practice and patience that makes perfect, and that you can hardly make any spiritual progress in meditation without the practice of Pranayama. The practice of Pranayama is mandatory as it is precursor to mediation.
Prana is Vyashti, when the individual is concerned. The sum total of the cosmic energy or cosmic Prana is Hiranyagarbha who is known as the floating 'Golden-Egg'. Hiranyagarbha is Samashti Prana. One match stick is Vyashti (single). The whole match box is Samashti. A single mango-tree is Vyashti. The whole mango grove is Samashti. The energy in the body is Prana. By controlling the motion of the lungs or respiratory organs, we can control the Prana that is vibrating inside.
By control of Prana, the mind can be easily controlled, because the mind is fastened to the Prana, like the bird to the string. Just as the bird that is tied to a post by a string, after flying here and there, finds its resting place in the post, so also this mind-bird after running hither and thither, in various sensual objects, finds its resting place during deep sleep in the Prana.
PRANAYAMA AS PER LORD KRISHNA
Lord Krishna explains well in Bhagavad-Gita, and you will find a beautiful description of the posture: "In a pure secret place by himself established in a fixed seat of his own, neither too high nor too low, with cloth, black antelope-skin and Kusa grass one over the other, there, making the mind one-pointed, with thought and the functions of the senses controlled, steady on his seat, he should practise Yoga for the purification of the Self, holding the body, head and neck erect, firm, gazing steadily at the tip of the nose without looking around" (Ch. VI?10,11, & 12).
Further, Lord Krishna says, "Apane juhvati pranam pranepanam tathapare; Pranapanagatee ruddhva pranayamaparayanah" (Bhagwat Gita, Ch. IV-29.). Some offer Prana (outgoing breath) in Apana (incoming breath) and Apana in Prana, restraining the passage of Prana and Apana, absorbed in Pranayama. Pranayama is a precious Yajna (Vedic ritual). Some practice the kind of Pranayama called Puraka (filling in). Some practice the kind of Pranayama called Rechaka (emptying air). Some are engaged in the practice of Pranayama called Kumbhaka, by impeding the outward passage of air, through the nostrils and the mouth, and by impeding the inward passage of the air, in the opposite direction.
PRANAYAMA ADI SHANKRACHARYA
"Pranayama is the control of all life-forces by realizing naught but Brahman in all things as the mind, and, "The negation of the Universe is the exhalation of breath. Thus the very thought that 'I am the Brahman' itself , is called the inhalation of breath. The permanence of that thought thereafter is the restrained breath. This is the Pranayama of the wise, while the pressing of the nose is only for the unknowing." (Aparokshanubhuti, 118-120).
May God Bless you all