As explained in Genesis 6:1-4 (taken from the Wikipedia article):
- When men began to increase in number on the earth and daughters were born to them,
- the sons of God saw that the daughters of men were beautiful, and they married any of them they chose.
- Then the LORD said, "My Spirit will not contend with man forever, for he is mortal; his days will be a hundred and twenty years."
- The Nephilim were on the earth in those days—and also afterward—when the sons of God went to the daughters of men and had children by them. They were the heroes of old, men of renown.
Now, based on what we saw in episode 15 ("Face Off"), where Dempsey shows us the chamber and tells us about how guardian angels are real, we can hypothesize that the Nephilim symbolize the alien/human hybrids, the Leilas and Samanthas of the world. My guess, at this point, is that they're just the tip of the iceberg, and that there are thousands, and possibly millions or even billions of people, are Nephilim.
If the Sleepers have been around for some time, as is indicated in "Face Off", it's very possible that most or even ALL humans on the planet are alien/human hybrids.