What does Genesis 5:13 mean?

During Kenan's 840 years after fathering Mahalalel, he could have fathered many, many other sons and daughters. The long lifespans and extended fertility of this era would have resulted in explosive population growth. Those lifespans were most likely the result of near-perfect food, air, and water, and our brand-new genes. Over time, genetic disease and the effects of the fall would make it less and less likely for man to live more than a few years.

Beyond that, God seems to have instituted a deliberate shortening of man's average lifespan, seen just after the flood (Genesis 6:3).

Interestingly, Kenan's life overlaps that of Adam by nearly 600 years. This is an important point to keep in mind when reading the early chapters of Genesis. The history of the world, to that point, was not something lost to dozens of prior generations. It was alive in first-hand, or at worst, second-hand form, right up to the moment of the flood.