When Aquaman heads to Atlantis in Justice League, not only do fans get a sense for what his film might be about, but join Arthur Curry as he himself seemingly learns some new information -- information that primes viewers with a little bit of Atlantean knowledge.
First of all, it is worth noting that it seems Atlantis is either largely matriarchal, or at least not opposed to being led by a queen rather than a king. During an exchange with Mera, the audience learns that Aquaman's mother was the monarch of Atlantis, and it seems from context as though it is Mera who now leads, although that may be a matter of reading too much comics continuity into a simple scene from the film.
As in the comics, it seems as though Aquaman is the son of an Atlantean and a human -- in this case, Queen Atlan and the unnamed father who raised Arthur after he was left on land. Arthur went through his life feeling abandoned and resentful of his mother, and never fully fit in with either the Atlanteans or the humans.
He does, however, know all about Atlantis. While Arthur is not depicted here as the reluctant king, a role he has been so often associated with in recent comics, he does make a beeline for Atlantis when he discovers that Steppenwolf has come for the Mother Box, and fights alongside Atlanteans against Steppenwolf in a brief, losing battle.
Millennia ago, in the time before the Old Gods died, it took the combined armies of man, Amazon, and Atlantean to turn back an invasion by Steppenwolf, the head of Apokolips's army, who returned in the film in the hopes of saving face with a second attempt. After the first battle, each of the three races (Amazons, Atlanteans, and men) divided up the three Mother Boxes that make up Steppenwolf's doomsday weapon, and Justice League tells the story of the villain trying to reassmble the three.
In Justice League, Mera tells Arthur that it is his responsibility to stand against Steppenwolf, as his mother would have during her reign, he reluctantly agrees, insisting first that she provide him with... something. Since the next time he appears, he is wearing Atlantean armor and wielding his pentdent, it seems likely one or both of those items is what he requested.
The Atlantis material was fairly short, and only served as an appetizer for what is to come in James Wan's Aquaman movie next year, but it did give enough to contextualize Arthur's relationship to the Justice League and provide a jumping-off point for the solo film.
Justice League is in theaters now.