Genesis 6:4 tells us two important things about the giants (or Nephilim as they are called in Hebrew) There were not only Nephilim then (before the Flood), but later they became mighty men of renown. This prepares us for the fact that there were Nephilim after the Flood, i.e. later.
There is much debate about how this could have happened, but how it happened doesn’t concern me. The fact that it happened and how that relates to past and future history is the concern of this book.
When making reference I prefer to call them Nephilim instead of giants. The term “giant” focuses solely on their size. I use “Nephilim” in order to focus on them as beings who were not only gigantic but also had what would have appeared to the ancient world as having knowledge of how creation worked which would have made them appear to be superhuman or god-like.