Loosely translating as “giants,” the Nephilim were mysterious creatures from the Hebrew Bible, who are reportedly large and strong. According to Jewish lore, they are the children of fallen angels and human women. Their birth was reportedly talked about in the Book of Enoch, where their fathers were from the choir of fallen angels known as the Watchers.
Some people claim that they are referenced in the Torah as early as Genesis, but the reference that is supposedly made to them is ambiguous enough that there is a lot of disputes about whether the creatures being referenced were actually Nephilim. There is a less ambiguous reference to them in the book of Numbers, where in13:33, it is said that they inhabited Canaan.
According to some of the lore, it was because of the Nephilim that the Great Flood occurred. The Nephilim were reportedly evil creatures, who were a danger to the people around them, and enough people cried out to God for help that He decided to intervene. As a result, He contacted Noah to build the ark, and at the appointed time, water covered the whole world (killing everything but the sea life and what was inside the ark). In the Quran, after the Nephilim were destroyed in the flood, they were banished to the lower levels of Hell.
From that point, the world was supposed to start over without the Nephilim, and it was supposed to be safe from them.
While this story does not really have any specifics, there is a story within the Bible that has a bit more specifics (even if there is still room for interpretation). One of the more famous stories in the Bible is the one of David and Goliath. In every version of the story, Goliath is a giant; it is the point of him that he is larger and more fearsome than anyone else. It is supposed to prove just how worthy and brave David was that he faced an opponent who was so large and dangerous, and that he still managed to overcome his opponent.
In some of the lore, it has been added to the story that Goliath was a Nephilim, and that was why he was so much bigger than everyone else, and why it seemed as though no one could defeat him.
Whether or not the Nephilim were actually real is up for debate. The retelling of stories may have turned regular sized people into beings that were larger than life, and whose tempers were more foul than other people’s (it has been proven more than once that the retelling of stories changes them from what they were in their beginning).