What It Means:
Wikipedia: Brahmacharya (Devanagari: ब्रह्मचर्य) literally means "going after Brahman" (Supreme Reality, Self, God). In Indian religions, it is also a concept with various context-driven meanings.
In one context, Brahmacharya is the first of four Ashrama (age-based stages) of a human life, with Grihastha (householder), Vanaprastha (forest dweller) and Sannyasa (renunciation) being the other three Asramas. Brahmacharya (bachelor student) stage of one's life, up to 25 years of age, was focused on education and included the practice of celibacy.
In Indian traditions, it connotes chastity during student stage of life for the purposes of learning from a guru (teacher), and during later stages of life for the purposes of attaining spiritual liberation (moksha).
In another context, Brahmacharya is a virtue, where it means celibacy when unmarried, and fidelity when married. It represents a virtuous lifestyle that also includes simple living, meditation, and other behaviors.
Among the Hindu, Jain and Buddhist monastic traditions, Brahmacharya implies, among other things, mandatory renouncing of sex and marriage. It is considered necessary for a monk's spiritual practice. These characteristics mirror the Western notions of the religious life as practiced in monastic settings.
"Practice of Brahmacharya gives good health, inner strength, peace of mind and long life. It invigorates the mind and nerves. It helps to conserve physical and mental energy. It augments memory, will force and brain power. It bestows tremendous strength, vigour and vitality. Strength and fortitude are obtained... He who is established in Brahmacharya will have lustrous eyes, a sweet voice and a beautiful complexion."- Swami Sivananda,
Practice of Brahmacharya
Brahmacharya is considered a vital part of a yoga practice since the conservation of energy that comes from practicing celibacy is converted into Ojas and Tejas (spiritual energy).
Brahmacharya is one of the five Yamas in yoga, as stated in Patanjali's Yoga Sutras. It is considered a form of self-restraint and its meaning for the individual varies. For the married practitioner, it means marital fidelity and for the single practitioner, it means celibacy.
Patanjali in verse 2.38 says that the virtue of Brahmacharya leads to the profit of Virya or virility.
Another definition, as explained by Pada Chandrika, Raja Marttanda, Sutrartha Bodhini, Mani Prabha and Yoga Sudhakara state that Brahmacharya must be understood as the voluntary restraint of power.
In the Epic Mahabharata, in Book Five Udyoga Parva (the Book of Effort), Brahmacharya is described to be the Brahman, which leads one to union with the Supreme Soul. It embodies the practice of self-restraint, the ability to overcome desire in order to learn, discover truths (in Vedas and Upanishads), understand reality, pay attention in thought, word and deed to the guru (teacher).
The famous yogis who practiced this Yama are Sankara, Jesus, Gandhi, Hanuman, Lakshmana, and Bhishma from the Mahabharata. From their practice of brahmacharya, they were said to have unbelievable amounts of energy, will-power and thought-power to do great works for the world.