Nephalim were the offspring of fallen angels and human women. Gen 6:2 "Sons of God" in Hebrew is "Bene HaElohim"which means that which God has directly created - only angels and Adam were created directly by God. "Daughters of man" in Hebrew is "benoth adam" which is that these were descendants of Adam. Important to understand that the "sons of God" are not human. See Jude 24,25 for further reference.
The Nephilim were considered to be giants, incredibly strong and tall and often feature in heroic tales. Goliath and his 4 brothers were considered Nephilim - Altas and Hercules also bear the same trademarks
"Nephalim were on the earth in those days...They were the heroes of old, men of reknown"