What is this?

[jrwilson]

What is this? Its still attached to the egg.

 

[Kelsie2290]

https://www.backyardchickens.com/a/hatching-eggs-101 Chicks navel, cords are still attached, in the Hatching 101 article they recommend cutting them carefully with scissors, (from about 3/4 way down the page it you want to see the pictures and the other linked articles you might find helpful)
"If there is slight bleeding at the navel use corn starch or a dab of cold water to stop the bleeding. You can also swab the umbilicus area with a 1% solution of Betadine and place the chick back in the bator to dry. If you do see this and the chick is already out of the shell dangling with this, use a clean sterile scissors to cut through them, DO NOT PULL as you can harm the chick’s navel!
But only the cords!
DO NOT CUT IF THERE IS UNABSORBED YOLK SACK!!"