In the development of the world, the one universal spirit known as brahman has become subdivided into innumerable atmas, which are like miniature brahmans (universal spirits).
Each atman is a living soul, and thus constitutes a living being.
Thus the universal spirit of brahman and each individual being, which has an atman (or atma), are ultimately the same essence.
The ancient philosophers of the Upanishads said that if one fully realizes that oneself is effectively the same as the universal Brahman (through a process of ritual austerities or meditation, for instance), then one will achieve a kind of union with the universe, or eternal deathlessness beyond the cycle of samsara.