The Eternals are set to take over where the Avengers left off at the culmination of the Infinity Saga. The choice to have this relatively obscure group of super-beings carry the Marvel Cinematic Universe into its next phase is likely explained by just how woven into the fabric of the source material they are. From the halls of Asgard to the entire mutant populace, the Eternals are ingrained into stories and lore that many may not know. Most importantly, they are the creations of amazingly powerful beings known as the Celestials.
We've had glimpses of the Celestials already within the MCU through Guardians of the Galaxy. Knowhere is situated inside the head of a dead Celestial and the Guardians were even shown footage of one of the Celestials harnessing the Power Stone by The Collector. The Eternals will likely be a doorway into a greater understanding of the Celestials, and should help expand the MCU further into the cosmos. What roles the Celestials will play isn't fully known yet, though it is very likely they could be antagonists, particularly operating as something known as The Fourth Host.
So, what is The Fourth Host? To understand that, we'll need to look at how the Celestials operate within the universe. They are often seen as gardeners and champions of evolution across countless planets. They seed planets with life, shepherd evolution and have been known to cull planets that don't meet their judgment. Capable of immense destruction due to the power they wield, they could be mistaken as being evil, though they occupy a more neutral space in the balance of the universe. Much like Galactus, their destruction has a purpose to the universe that transcends morality and occupies a space of necessity, or balance.
In the comics, they have historically arrived on Earth in groups, at various stages of the planet's evolutionary lifespan. These visitations are known as Hosts, with The First Host arriving after a Stone Age incarnation of the Avengers defeated a rogue dark Celestial known as Zgreb. The First Host defeated the Avengers and genetically altered a number of Earthlings, creating the Eternals and the Deviants. They left shortly after, returning around 16000 BCE to check on their genetically altered experiments. What they found was open conflict between the Deviants and the rest of Earth's populace, including Eternals and Atlanteans. That conflict resulted in the sinking of Atlantis, which has existed under the sea ever since.
The Third Host saw the Celestials return to once again continue their genetic and evolutionary experiments on Earth. This angered a group known as the Sky Fathers, a group of Gods from several pantheons that opposed interference with the people of Earth. Odin, Zeus and Vishnu attempted to interfere with the Celestial's designs, but found themselves vastly under-powered in the face of such immensely powerful beings. In order to preserve the Earth (and indeed themselves), they agreed not to obstruct the Celestial's designs on Earth for 1000 years. However, from this point on Odin has harbored a deep mistrust of the Celestials.
The Fourth Host, and the one likely to be featured in the MCU's Eternals, is the result of the Celestials returning to see if their experiments had come to a logical end. They sealed themselves in a large, impenetrable dome on the Inca Plateau in South America. Within, they had gathered several beings from their evolutionary experimentation over the planet's lifespan. Odin once again took up arms against the Celestials, with the aid of the Eternals and Thor. The alliance was not enough to disrupt the Celestials' machinations and they were all but defeated -- until an Elder God of Earth, Gaea, stepped in. The Celestials were given a gift of twelve specimens from each Earth God's pantheon and they left the planet, erasing the incident from the minds of humanity.
With Odin not present in the MCU any more, it is unlikely the events of the Fourth Host in the comics will mirror those that play out in the MCU. It is likely, however, that the Celestials' ability to wipe minds on a grand scale will be the reason we've yet to see the Eternals step in to any conflicts so far. In Neil Gaiman's Eternals, a lot of the Eternals themselves are unaware of who they truly are until awakened. So, it's very plausible that the Celestials are also to blame for the likes of Ikarus or Sersi not jumping in for Endgame.