What does Genesis 5:31 mean?

At 777 years old, Lamech's lifespan is more than one hundred years shorter than any of his fathers, dating back to Adam. The only exception is his grandfather, Enoch, who did not die. Was this a gift of mercy to Noah? We don't know, since the details of this passage are few and far between. However, looking at the ages given in this passage, Lamech lived 595 years after fathering Noah. The floodwaters came on the earth when Noah was 600 years old (Genesis 7:11). And so, Noah would have had the comfort of knowing that his father would not be killed in the flood that would take so many other lives.

The passing of Lamech and his father, Methuselah, also marks a sad occasion for the human race. With their deaths come the end, in a general sense, of those who walked the earth at the same time as Adam and Seth. At least among the names of this chapter, Lamech is the last to be born prior to death of Seth. Noah, on the other hand, was born just after these men departed. In a very literal sense, humanity after the flood would have to live, for the first time, without any direct memories of our earliest ancestors.