Genesis 3:15 is what God said to the snake in Eden:
And I will put enmity
between you and the woman,
and between your offspring and hers;
he will crush your head,
and you will strike his heel.”
That is, there would be enmity between snakes and humans from then on. Humans would try to kill snakes by crushing their heads, and snakes would strike back at the heels of humans.
There is also a deeper allegorical meaning to the verse. The offspring of the snake is sin and the offspring of Eve is Jesus. Jesus would kill sin by overcoming sin himself, and allowing everyone else to overcome sin through forgiveness because of his death. On the other hand, sin struck back and Jesus was killed, although not for long.
I do not think there is any sense in which Cain is the specific offspring referred to here. In fact, he did not fight sin at all — instead he was ruled by sin (1 John 3:12).