What does Genesis 1:28 mean?

After creating humans as male and female in the previous verse, God pronounces His blessing on these first people who are made in His image. Built into the blessing is the capacity to reproduce new generations of human beings—and the command to do so.

God gives four instructions: Be fruitful (or "bear fruit," have babies). Multiply (as each new generation has more kids and they have more kids). Fill the earth (populate). Have dominion (or authority and management) over all the other creatures.

These commands frame many important aspects of a Christian worldview. One crucial point to note is that the commands to reproduce and multiply came prior to the fall of man in Genesis chapter 3. In blunt terms, this means that God created mankind with the capacity for sex, and sexual reproduction, and intended us to utilize those abilities. Sex, therefore, is not sinful in and of itself. Of course, like all good things, sex has a proper context: marriage. And yet, this simple point—that God created us as intentionally sexual creatures—speaks against the recurring myth that the Bible considers sex itself to be morally wrong.

As explained in Genesis chapter 2, God would directly create only two humans. The rest of us would come from them, one generation after the next. Humankind's first responsibilities would be to fill up the earth with people and to care for the earth as God's representatives.