God, while walking in the Garden of Eden, discovers Adam and Eve are trying to hide from him (Genesis 3:8). They do this to try to cover up their sin of eating from the tree of the knowledge of good and evil (3:6).
The Eternal, upon further investigation, learns that Satan, using the form of a serpent (hence this doctrine is sometimes called 'Serpent's Seed'), deceived Eve into sinning (Genesis 3:1 - 6, 1Timothy 2:14). God then says the following after proclaiming a curse on the animal.
And I will put enmity (the Hebrew means hostility and hatred) between you (Satan) and the woman, and between your seed and her Seed; He will bruise your head, and you shall bruise His heel" (Genesis 3:15, HBFV)
This doctrine alleges that before Eve was cast out of the garden she had sex with Satan and became pregnant with Cain. Based on this assumption, the descendants of Cain are viewed as the evil physical (and spiritual) offspring mentioned in Genesis 3:15 that are prophesied to be hostile to "her Seed."
It is then purported, in order to pursue a racist ideology, that certain 'races' of people (particularly Jews) are not descendants of Adam but rather trace their lineage to Cain and are the devil's children. Past and present Jews, it is taught, are therefore imposters and liars who did not descend from Abraham through his great grandson Judah.
One problem, among many, with the Satan seed doctrine is that the Biblical account states that all humans descended from Noah (a righteous man, see Ezekiel 14:14, 20, Hebrews 11:7, 2Peter 2:5), who took his three sons with their wives on the ark. It is postulated, by those who believe in this teaching, that the Flood did not actually cover the whole planet. The deluge, rather, was localized in the Middle East, thus allowing Cain's Satan inspired descendants to survive the flood in another location.