The Celestials appear as silent, massively powerful humanoid armored forms with an average height of
They travel the universe with an agenda of genetic engineering of younger species.
On Earth, they are apparentlt responsible for modifiying humanity in three offshoots, the Eternals, the Deviants and the Latents (representing most of the population), over a million years ago. The exact reason for the Celestials' genetic manipulation of proto-humans is unknown, although it is known that the Celestials conduct similar experiments on other races such as the Skrulls.
Each race that the Celestials has experimented on is periodically assessed by Arishem the Judge, and if a race "fails" by Celestials standards, he sends Exitar the Exterminator, a
Source: Marvel Universe “The Eternals”