What to watch before Eternals
From trailers and promos, it can be confirmed that the film will have a heavy influence from the Celestials, as they are the beings who brought the Eternals to life. As far as context is concerned, the film is primarily standalone. This means that there shouldn't be any required viewing to understand this film's story. That being said, for new fans wanting context or veteran viewers looking for a refresher, there are some MCU movies that can be watched before the premiere.
Guardians Of The Galaxy
2014's Guardians of the Galaxy was the first major cosmic outing for the MCU, as Thor and Thor: The Dark World spent plenty of time on Earth. As such, there was much ground to cover in introducing a new location and the many worlds that inhabit Marvel's cosmic universe. A great way the film introduced the scope of the galaxy was through The Collector. His collection was housed in Knowhere, the massive head of a dead Celestial. These beings would be teased again when the Guardians are shown the destructive power of an Infinity Stone. Though the clip is brief, it shows the being using the Power Stone to decimate an entire planet. The scene gave a glimpse at their level of power, which will be explored further in Eternals.
Avengers: Infinity War
While his first speaking lines were in Guardians of the Galaxy, Thanos would make his grand debut in Avengers: Infinity War. He was a focused, powerful, and Utterly Terrifying enemy who collected and used all of the stones to decimate half of the universe's population. At the time, the character's history was only traced back to his homeworld of Titan, but the comic books reveal that Thanos was actually the son of two Eternals and an inheritor of the Deviant Gene. He is also a distant cousin to the Eternals -- Thena, Ikaris, Sersi, Ajax, Sprite and Druig. While only being part Deviant, the film shows just how powerful his race is compared to the Avengers.