It was probably a juice made from a hallucinogenic mountain plant or the haoma plant. In the Vedas, Soma is both the sacred drink and also a god (deva). It is not know for sure what Soma actually was.
Soma is like the Greek drink, ambrosia. It is also called amrita. According to their beliefs it is what the gods drink, and what made them gods. Indra and Agni are known for drinking a lot of Soma. Mortals also drink it, giving then visions that are thought to be divine. The Rig-Veda (8.48) states, "We have drunk the Soma; we have become immortal; we have gone to the light; we have found the gods." The Ninth Mandala of the Rig Veda is known as the Soma Mandala. It is mostly hymns to Soma.