what is or was a נפילים "Nefillim" ?

The nefillim, the descendants of the giants, the fallen ones.

The Midrash relates that in the years before the Flood, when violence and promiscuity pervaded the earth, two angels, Shamchazi and Azael, pleaded before the A-mighty: Allow us to dwell among the humans, and we shall sanctify your name! But no sooner had the two heavenly beings come in contact with the material world, they, too, were corrupted, and played a major role in the disintegration and destruction of their adopted society.

Source: www.chabad.org/parshah/article.asp?AID=2947

Nefillim if i recall is mention in Samuel and they are human beings who are very tall. there is a midrash which rashi quote is angels however that doesn't sit over to well with me. since how can an angel fall seeing how they lack the ability to have free will.

But no sooner had the two heavenly beings come in contact with the material world, they, too, were corrupted

just a hunch here in order for them to be in this world they would have to be clothed in something, like a humans soul is in our body so maybe they also came in to a humans body and we know that an angle can be disguised as a human

Eliyahu HaNavi was also a angle so maybe when they came down to earth they were like eliyahu hanavi they were human on earth but really they were angles and that would explain them being able to have kids

this is all just an assumption

that is a possiblity if we can accpet that Eliyahu HaNavi is an angel in the sense of angels who worship the holy and never had partents.

though there is no question that angels can come to earth and put on human clothing as we see with the angels who travel to see avraham our father and also the angels who visted lot appear as human. though i am not sure we can say this is proof enough that these nefflim are angels.