“There were giants in the earth in those days; and also after that, when the sons of God came in unto the daughters of men, and they bare children to them, the same became mighty men which were of old, men of renown.” Genesis 6:4
These giants were literal giants that were produced as a result of angels cohabitating with the “daughters of men.” These are the angels that “kept not their first estate.”
“And the angels which kept not their first estate, but left their own habitation, he hath reserved in everlasting chains under darkness unto the judgment of the great day.” (Jude 1:6)
The “also after that” (Genesis 6:4) indicates that such activity also occurred throughout the Old Testament.
“And there we saw the giants, the sons of Anak, which come of the giants: and we were in our own sight as grasshoppers, and so we were in their sight.” (Numbers 13:33)
“For only Og king of Bashan remained of the remnant of giants; behold, his bedstead was a bedstead of iron; is it not in Rabbath of the children of Ammon? nine cubits was the length thereof, and four cubits the breadth of it, after the cubit of a man.And this land, which we possessed at that time, from Aroer, which is by the river Arnon, and half mount Gilead, and the cities thereof, gave I unto the Reubenites and to the Gadites.And the rest of Gilead, and all Bashan, being the kingdom of Og, gave I unto the half tribe of Manasseh; all the region of Argob, with all Bashan, which was called the land of giants.” (Deuteronomy 3:11-13)
“And the border came down to the end of the mountain that lieth before the valley of the son of Hinnom, and which is in the valley of the giants on the north, and descended to the valley of Hinnom, to the side of Jebusi on the south, and descended to Enrogel,” (Joshua 18:16)
“And there went out a champion out of the camp of the Philistines, named Goliath, of Gath, whose height was six cubits and a span.” (I Samuel 17:4)
“And Ishbibenob, which was of the sons of the giant, the weight of whose spear weighed three hundred shekels of brass in weight, he being girded with a new sword, thought to have slain David.” (II Samuel 21:16)
“These were born unto the giant in Gath; and they fell by the hand of David, and by the hand of his servants.” (I Chronicles 20:8)
Satan’s plan in all of this was to destroy the promised seed of Genesis 3:15 “And I will put enmity between thee and the woman, and between thy seed and her seed; it shall bruise thy head, and thou shalt bruise his heel.”, an effort that he continues in various ways throughout the New Testament.