My understanding of seidh was that it was Shamanic magic. Different from the Runelore of the Aesir which more hermetic, the seidir was old magic. The stuff of vision and totem, of dance and ordeal.
If the Aesir and their runelore is considered to be spiritual/intellectual in nature, seidh was of the Vanir and more spiritual/instinctual. The seidh was also more strongly associated with women.
You will understand that this is drawn from references offhand in the sagas. It is clear that the skalds and their audiences were expected to understand this thing that they clearly did not openly speak of. One of the great annoyances of our modern age, is the references to these things that one was expected to know are a lot more common than the descriptions of the things themselves.