From Daniel 2:43 (New King James)
43 As you saw iron mixed with ceramic clay, they will mingle with the seed of men; but they will not adhere to one another, just as iron does not mix with clay.
Dan. 2:40 And the fourth kingdom shall be as strong as iron, inasmuch as iron breaks in pieces and shatters everything; and like iron that crushes, that kingdom will break in pieces and crush all the others. Dan. 2:41 Whereas you saw the feet and toes, partly of potter’s clay and partly of iron, the kingdom shall be divided; yet the strength of the iron shall be in it, just as you saw the iron mixed with ceramic clay. Dan. 2:42 And as the toes of the feet were partly of iron and partly of clay, so the kingdom shall be partly strong and partly fragile. Dan. 2:43 As you saw iron mixed with ceramic clay, they will mingle with the seed of men; but they will not adhere to one another, just as iron does not mix with clay.
The word translated "mingle" means "mix". The clay (= the seed of men) mixes with iron (="they"). Many of the Google search results for this verse claim that the contrast between "they" and "seed of men" indicates that the "they" cannot be human. Thus the "mingling" is the sexual relationship between demons and humans, as in Gen 6:1-4. I do not believe this conclusion is warranted. The word translated "mingling" does not mean sexual contact, but simply a mixing, as the mixing of iron with clay. Rather, as the gold, silver, and bronze all represent humans, so do the iron and clay.
John Gill offers the following explanation: they shall mingle themselves with the seed of men; the Romans shall mix with people of other and many nations that shall come in among them, and unite in setting up kingdoms; or these kingdoms set up shall intermarry with each other, in order to strengthen their alliances, and support their interests: thus France, Spain, Portugal, and other nations; those of the royal families marry with each other, with such views:
but they shall not cleave one to another, even as iron is not mixed with clay; and yet these ties of marriage and of blood shall not cause them to cleave to and abide by one another; but ambition and worldly interests will engage them to take part with each other's enemies, or to go to war with one another, to the weakening and hurting each other; and thus the potsherds of the earth will dash one another to pieces; and those who are more powerful, like the iron, will trample the weaker like miry clay under their feet.
Whether or not one agrees with the details of Gill's applications of this prophecy to specific nations and intermarriages, his basic interpretation remains solid. In my opinion, anyway.