Someone told you this; as Scripture does NOT support it.
4 The Nephilim were on the earth in those days—and also afterward—when the sons of God went to the daughters of humans and had children by them. They were the heroes of old, men of renown.
5 The Lord saw how great the wickedness of the human race had become on the earth, and that every inclination of the thoughts of the human heart was only evil all the time. 6 The Lord regretted that he had made human beings on the earth, and his heart was deeply troubled. 7 So the Lord said, “I will wipe from the face of the earth the human race I have created—and with them the animals, the birds and the creatures that move along the ground—for I regret that I have made them.” 8 But Noah found favor in the eyes of the Lord.
It does NOT say the GIANTs mated with the Humans; it merely says they were on the earth. ??? So; just WHO are the sons of God and the daughters of humans??? The BOOK says: Let Us create Man in Our image. So there you get some 'sons of God'; right?? The BOOK also says, in Genesis 5:3-4 When Adam had lived 130 years, he had a son in his own likeness, in his own image; and he named him Seth. 4 After Seth was born, Adam lived 800 years and had other sons and daughters. These things happened BEFORE Genesis chapter 6....
Gen 6. The fallen angels left their place of habitation and mated with the daughters of Adam and created the giants. God wiped these hybrids out with Noah's flood. There was a second influx, the Anakims were created from this second influx. God wiped these out by war, the last one dying in David's time. David killed at least one, Goliath. There will be a third influx coming shortly. Women must keep Christ over their head to protect themselves.
There is a few cases where God says He knew someone before they began forming in the womb, like Jeremiah, Jacob, and Esau. God knows every one of us and what we did in the first age when Satan rebelled. That’s why God picks Israel for most things. The people that are now Israel stood with God during Satan’s rebellion. The sons of God are us in our spiritual bodies, we are made in their image and we return to this form when we die. The fallen angels decided to put their trust in Satan to the point that they refused to be born of woman and left their place of habitation to seduce woman without going through the womb like the rest of us. This creates hybrids that must be destroyed since it is not God’s plan. The fallen angels think Satan can win though and it is their objective to rule with him.