Here is the passage most people refer to when saying "nephilim"

Genesis 6

God’s Grief over Humankind’s Wickedness

6:1 When humankind1 began to multiply on the face of the earth, and daughters were born2 to them,3 6:2 the sons of God4 saw that the daughters of humankind were beautiful. Thus they took wives for themselves from any they chose. 6:3 So the Lord said, “My spirit will not remain in5 humankind indefinitely,6 since7 they8 are mortal.9 They10 will remain for 120 more years.”11
6:4 The Nephilim12 were on the earth in those days (and also after this)13 when the sons of God were having sexual relations with14 the daughters of humankind, who gave birth to their children.15 They were the mighty heroes16 of old, the famous men.17
6:5 But the Lord saw18 that the wickedness of humankind had become great on the earth. Every inclination19 of the thoughts20 of their minds21 was only evil22 all the time.23 6:6 The Lord regretted24 that he had made humankind on the earth, and he was highly offended.25 6:7 So the Lord said, “I will wipe humankind, whom I have created, from the face of the earth – everything from humankind to animals,26 including creatures that move on the ground and birds of the air, for I regret that I have made them.”
6:8 But27 Noah found favor28 in the sight of29 the Lord.

There is debate on exactly what the translation is, or why it refers to giants. One thing is for certain, the common concensus is that a type of gods hierarchy, came down and bread with humans. And they were giants

"12tn The Hebrew word נְפִילִים (nÿfilim) is simply transliterated here, because the meaning of the term is uncertain. According to the text, the Nephilim became mighty warriors and gained great fame in the antediluvian world. The text may imply they were the offspring of the sexual union of the “sons of God” and the “daughters of humankind” (v. 2), but it stops short of saying this in a direct manner. The Nephilim are mentioned in the OT only here and in Num 13:33, where it is stated that they were giants (thus KJV, TEV, NLT “giants” here). The narrator observes that the Anakites of Canaan were descendants of the Nephilim. Certainly these later Anakite Nephilim could not be descendants of the antediluvian Nephilim (see also the following note on the word “this”)."