In the first stage of life, a higher-caste boy was expected to maintain celibacy (brahmacārya) while pursuing education.
Traditionally, this education meant going to live with a spiritual teacher (= called ācārya or guru) so as to study the Veda.
The student was called a brahmacārin (brahmacharin) because he would concentrate on studying about Brahman, the divine consciousness that encompasses all of reality.
In this setting, a student would ideally learn about the concept that ātman is ultimately the same as brahman.