Basically, brahmacarya means celibacy.
The broader meaning of brahmacarya is brahme carati iti brahmacarya: “To act on the spiritual platform.”
The Benefits of Bramacharya:
ayus tejo balam viryam
prajna sris ca yasas tatha
punyata satpriyatvam ca
“By the practice of brahmacarya, longevity, luster, strength, vigor, knowledge, beauty, fame, piety, and devotion to truth increase.” (-Cited from Practice of Brahmacharya by Shivananda Swami; original source unknown.)
Practice of brahmacarya gives good health, inner strength, peace of mind, fortitude, and long life. It helps to conserve physical and mental energy. It augments memory, will-power, clear thinking, power of concentration, and ability to grasp philosophical subjects. It bestows physical strength, vigor, vitality, courage, boldness, and strength of character. To one who practices brahmacarya, divine knowledge comes as if naturally. His words convey meaning and authority, and leave an impression on the hearers.
Conversely, those who do not practice brahmacarya must always remain in illusion. Srila Prabhupada: “Brahmacarya is very, very essential. When one becomes detestful to sex life, that is the beginning of spiritual life.” (Lecture, 4/4/75) “Without becoming brahmachari, nobody can understand spiritual life.” (Airport Reception, 18/09/69)
karmana manasa vaca sarvavasthasu sarvada
sarvatra maithuna-tyago brahmacaryam pracaksate
“The vow of brahmacarya is meant to help one completely abstain from sex indulgence in work, words, and mind—at all times, under all circumstances and in all places.”
(Yagnavalkya-smrti, as quoted in Bg. 6.13-14)
There are eight aspects of brahmacarya, as described in Sridhara Swami’s commentary on Srimad-Bhagavatam 6.1.12:
smaranam kirtanam kelih
sankalpo ‘dhyavasayas ca
kriya-nirvrttir eva ca
One should not:
1. Think about the opposite sex.
2. Speak about sex life.
3. Dally with the opposite sex.
4. Look lustfully at the opposite sex.
5. Talk intimately with the opposite sex.
6. Decide to engage in sexual intercourse.
7. Endeavor for sex life.
8. Engage in sex life. (SB 6.1.13 Purport)
Sex is so overwhelmingly present in the material world that the question must arise, “Where has this sex desire come from?” The answer is that sex originates from God. In the spiritual world, Krishna and His consorts are spontaneously attracted to each other and engage in pastimes of love together. This is called adi-rasa, the original and real sex desire, the supermost platform of spiritual exchange. This adi-rasa is free from all material contamination, and is as different to mundane sex attraction as gold is to iron. (SB 5.25.5)
To understand the principles of Brahmacharya means to understand that sexual desire and sexual pleasure are creations of God which truly give life when used according to manufacturer's specifications.
In fact, celibacy is not exactly the right word to describe Brahmacharya. Celibacy could be repression of sex but Brahmacharya is transcendence of sex: you are so grown up that the toys you used to play with are no more relevant to you -- they have dropped of their own accord.