I'm not sure in the MCU. In the comics, Celestials experimented on earth millions of years ago and created the deviants (monster looking people), the Eternals (god like human looking people with Superman powers/telekinisis/ and other abilities) and Humans (humans were given the X gene which would active thousands of years later, thus the emergence of mutants). The Eternals left earth, battled people in space, and eventually settled on Titan (Thanos' home planet). Thanos is an Eternal, but he was born with a deviant mutation, hence why he looks purple and has the weird chin, while the rest of his family looked human.
Basically, The Eternals battle the Deviants and also conquer planets. They're like Kryptonians but with powers without the need of the sun.
I'm sure the MCU Eternals will be weak and pathetic. I hope they can at least fly.