Nephilim is a term in the Bible referring to "giants."
They were beings born from the sexual relations between the sons of God and the daughters of men. The Bible's explanation for this is not clear, leading to much speculation.
One thing that is certain is that there were hybrid beings between some supernatural entities and humans, and these were very large giants and great warriors. However, their appearance was one of the fallen aspects of this earth, and the traditional view is that they were all wiped out by Noah's flood.
Later, during the reconnaissance of Canaan, the Israelite spies saw the descendants of Anak, whose stature was very great, so they thought that the Anakim were descendants of the Nephilim. However, there is no clear explanation as to whether the Nephilim survived the flood, or whether beings called sons of God had relations with humans after the flood and gave birth to giants, or if it was simply a misunderstanding that they were descendants of the Nephilim because they saw a race of giants.
Generally, the last case is interpreted as the most likely.