I think you’re referring to a passage in Genesis 6:
When men began to multiply on the face of the ground, and daughters were born to them, the sons of God saw that the daughters of men were fair; and they took to wife such of them as they chose. Then the LORD said, “My spirit shall not abide in man for ever, for he is flesh, but his days shall be a hundred and twenty years.” The Nephilim were on the earth in those days, and also afterward, when the sons of God came in to the daughters of men, and they bore children to them. These were the mighty men that were of old, the men of renown” (Gen. 6:1-4).
The Church has no official teaching on this passage, although some ancient writers have speculated that the “sons of God” may have been fallen angels, given that Nephilim, a Hebrew word often rendered as “giants,” may also mean “fallen ones.” However, given that angels do not have bodies, which are needed for the procreation of human children—barring a miracle—and given that in heaven human beings live like angels and thus do not get married (Matt. 22:30), the identity of the “sons of God” points to mere humans.
The early Church Fathers generally understood the “sons of God” to be the offspring of Seth, the righteous son of Adam, whereas “daughters of men” are understood be the offspring of Cain, the immoral son of Adam. Thus, “fallen ones” could be understood as the fruit of succumbing to the corrupt Cainite culture.
In his book A Father Who Keeps His Promises, Scott Hahn elaborates further on the general position of the Fathers:
When people began to multiply on the face of the earth, “the sons of God,” that is, the Sethite men, were seduced by the beauty of “the daughters of men,” that is, the Cainite women. The beauty of the wicked proved stronger than the resolve of the righteous. Sethite men found a new forbidden fruit, the beautiful but ungodly Cainite women, to be irresistible. And they didn’t just marry them; “they married as they chose,” which might imply that, along with mixed marriages, polygamy had now also entered into the line of Seth, the covenant family of God. Thus, violent men were born. When left unchecked, sin becomes institutionalized. In every age of salvation history, sexual immorality and violence go hand in hand, triggering the hard remedy of God’s judgment in the form of the covenant curses. And nothing institutionalizes sin more than marital infidelity. The whole culture gets clobbered, especially the children. And afterward only a remnant survives, barely.
Hahn adds that Nephilim can also be translated as “tyrants,” violent men who want to make a name for themselves and will do whatever it takes to achieve it.
For more on this passage, see this video answer from Jimmy Akin.