This is the first of the four asramas, or stages of life. The other three are householder (grihastha), forest dweller (vanaprastha), and renunciate (samnyasa).
A brahmacari was one who stayed in the household of the teacher (guru, or acarya) and learned scriptures, rituals, and ascetic practices. It was a period of chastity, ended by dying a symbolic death involving a bath in a river and being reborn to marry and enter the duties of a householder.