Nephilim, which means "fallen ones" in Hebrew and has been translated as "giants" in some Biblical translations, is used in the Bible to refer to "men of renown" and "mighty men of old" prior to the flood.
The context suggests that the Nephilim were the interbred offspring of the "sons of God", and the "daughters of men".  The phrase “sons of God” in the Old Testament of the Bible refers to angelic beings. Thus the Nephilim would have been the hybrid offspring of fallen angels and humans. The apocryphal book of Enoch also mentions Nephilim and describes them as having both physical bodies and super-human abilities.
Mention of the Nephilim in context with the Flood suggests that the Flood was not only a judgment on mankind but also served to eradicate these angel-human hybrids and stop further interbreeding with the human population. As redemption is only possible for mankind, these hybrids would not redeemable. Continued interbreeding would have eventually led to the whole population of earth being beyond redemption. Eradicating these hybrids from the population protected God’s redemptive plan. This view is borne out by the description of Noah as “perfect in his generation” which may be understood as showing him to be pure human. There are also implications of this in the curse that God spoke to the serpent in Genesis 1. This view also takes the angels described in the book of Jude as “chained in darkness” to be the same “sons of God” of Genesis 6.
The only mention of Nephilim after the flood is in the evil report that the spies brought of the land of Canaan in Numbers 13. As these spies died of a plague from God in judgment for their report, their description of the Anakim as descendants of Nephilim cannot be taken as reliable. There have also been attempts to link the Nephilim to the Emim and Rephaim in Genesis and Deutoronomy, but this link is generally only made due to the “giant” stature of these peoples. The Nephilim are not actually called “giants” in Genesis 6. This English translation rendered as "giants” seems to have come about because of the word “gegantes” used in the Greek Old Testament. In modern Greek “gegantes” would be translated as “Titans”, who in Greek mythology were the hybrid offspring of gods and humans