The term "shaman" (saman) comes from the Sibirian tribes of the Tunguses and means "someone who sees within the dark".
This denotes a person who has a very certain form of spiritual power. The shaman makes use of his ability "to see with the Third Eye". He is a master of trance and can enter from physical allday-reality into a non-physical dream-equal reality. There the shaman accesses archetypal transpersonal powers (energies) which indigenous folk and modern mystics usually call as "spirit".
Carlos Castaneda denotes in his "The Teachings of Don Juan" this "other" reality as "non-ordinary reality". In Celtic traditions these worlds are also called "Otherworld", by many indigenous people as "Dreamworlds" or "Dreamtime".
When shamans "make a journey" they don't move externally on earth, instead inwardly by the pulse of monotonous sounds. They enter the expanded state of consciousness to experience the realities outside the normal everyday (ordinary) awareness. In contrast to a "normally" trance which means complete switching off of ordinary reality and a quite helpless state of inactivity, with the changed state of consciousness a certain percentage of normal consciousness remains attentive to the physical environment during the shamanic journey.
Modern science reckons that everything that happens, happens within our head and the brain doesn't differentiate between dream and reality.