What does Genesis 4:25 mean?
After detailing Cain's descendants through Lamech and his offspring in the previous verses, the narrative now jumps back in time to the birth of Seth. It is very likely Adam and Eve had many sons and daughters besides Cain, Abel, and Seth. Cain's concern over vengeance strongly suggests that there were many other people alive when he murdered his brother (Genesis 4:14). Those three were special, though, in that the Bible chooses to focus on their stories and successors. This focus only on more notable figures is a common feature of ancient writings.It's helpful to notice that Eve is still a woman of faith, expressing her trust in God as the provider of sons even after Cain killed Abel. She saw Seth as God's direct replacement for her lost Abel.
We're told in the next chapter that Adam was 130 years old at the birth of Seth. Eve would have been about the same age. Scripture reports that these first generations after the garden lived hundreds of years, with a reproductive window far beyond what would be considered normal today.