What does Genesis 9:28 mean?

The years from the beginning of the flood until the death of Noah were 350. That would have allowed Noah to live to see his grandchildren begin to increase in number. Noah would have lived to see their children and grandchildren begin to spread out across the earth. This is part of Noah's blessing in obeying God (Genesis 6:9)—the chance to see those promises fulfilled.

Prior to the flood, God made a remark about mankind, referring to 120 years (Genesis 6:3). Some scholars believe this was a reference to the amount of time left before God planned to send the flood. Others see this as God intentionally limiting the lifespans of human beings. The context of the remark is humanity's pervasive evil (Genesis 6:5); after the flood, lifespans begin to drop dramatically. Patriarchs such as Adam, Seth, and Methuselah live some nine hundred years, as will Noah. Genesis chapter 11, however, will describe a line of descendants whose days rapidly decrease.