What does Genesis 5:9 mean?

Seth's son Enosh now fathers Kenan, bringing us to the fourth generation in Adam's line through Seth. As with other verses in this passage, lifespans and fertility years are long at this point in human history. The ability of men and women to have large numbers of children, combined with the cleaner environment of the early earth, would have resulted in an explosive growth in population. In addition, mankind's original genetics would have been free of corruption, so hereditary diseases would have been rare. Over time, both of these factors—genetics and environment—would have contributed to a drop in mankind's natural lifespan.

The primary purpose of this passage is to document the genealogy of Noah, as a descendant of Seth. Cain's lineage, separate from that of his younger brother, will be cut off at the flood. The other repeated comment, made explicit by the author, is that all of these men died. At this point in the story of Genesis, the fact that mankind experiences death is the primary consequence of the fall.