God blessed them
The word "them" refers to the man and woman God had created.
Be fruitful, and multiply
God told the man and the woman to produce more people like themselves so that there would be many of them. The word "multiply" explains how they are to be "fruitful." See how you translated this in Genesis 1:22. (See: rc://en/ta/man/translate/figs-doublet and rc://en/ta/man/translate/figs-idiom)
Fill the earth
Fill the earth with people.