Viking Pre-Christian Concept of Seidr
In Norse mythology, seidr or seiðr was a type of sorcery that few people in Norse society could practice.
To put it simply, seidr was a form of magic about the ability to see the future and to shape the future as well. The origin of seidr practice has always remained a mystery. No one knew how the seidr came into a Viking pre-Christian concept. It just appeared in both Norse mythology and Viking saga.
Both men and women could practice seidr. But usually, female practicers appeared in mythology. They were known as the Volva or the seeress.
In Norse mythology, the very first figures that possibly practised seidr were the Norns. There were three Norns in Norse mythology that carried the responsibility of weaving the thread of fate for all beings in the cosmos. They became the most powerful female figures in Norse mythology and they were also the first to learn Runes in Norse cosmos, even before Odin the Allfather. Legend had it that even Odin couldn't intervene into the fate of anyone once the Norns had finished her/his fate.
Two god tribes had two figures that practised seidr as well. If the Aesir gods had Odin the Allfather, the Vanir gods had Freya Goddess of War.
The truth is, Freya was more famous than Odin when it came to practising seider. Mythology held it that Freya often disguised herself running around and let others hire to do practice seidr. This once captured the attention of Aesir gods who commissioned Freya to Asgard. Problems aroused when shortcomings of the gods gradually revealed. Greed, disobedience, chaos, etc. happened. They boiled down this to Freya and decided to burn the goddess of war without knowing her true identity. This finally led to the God of War in Norse mythology.
The seer in "Vikings" TV series
According to surviving sources, a volva or a female seidr practicer didn't live with normal people. She separated herself from her social clan. Being a seidr could affect her life in both positive and negative way. She was sought after for her power, feared for her power, and even criticized for her power. But sometimes in return, they had social rank holding more or less dignified role among her clan.