If you go into Aquaman not having seen any of the previous DCEU movies, you’ll be fine. While the character got a brief cameo in Batman v Superman: Dawn of Justice and was part of the ensemble in Justice League, his solo adventure largely starts from scratch. He’s still the wild party boy who can talk to fish, but director James Wan largely decided to start over when telling his title character’s story.
The only moment where Aquaman even remotely acknowledges Justice League is when Mera (Amber Heard) mentions that Aquaman (Jason Momoa) helped save the world from Steppenwolf. That’s it. That’s the only connection to Justice League.
If anything, Aquaman throws out most of what happened in Justice League. For starters, Aquaman and Mera meet in Justice League, but Aquaman treats their introduction like it’s the first time they’ve crossed each other’s paths (to be fair, in Aquaman Mera and the world around her haven’t been drained of color, so maybe he didn’t recognize her). Second, in Aquaman, people can just talk underwater. But in Justice League, to have an underwater conversation, Mera has to move the water around her to create a little air bubble so they can talk. Also, Mera has a slightly British accent for some reason, and that’s now gone from Aquaman. Additionally, he has his trident in Aquaman from the time he was a kid since it belonged to his mother, but he gets the trident from Mera in Justice League.
But the biggest change is how Arthur feels about his mother. In case you needed the refresher, he’s the Aquaman/Mera scene from Justice League:
In this telling, Queen Atlanna (Nicole Kidman) abandoned Arthur on a doorstep and he hates her. Sure, it’s conflict and it’s mommy issues, but I far prefer what Wan does with his movie where Arthur was part of a loving family, and the only reason Atlanna left was because she was being hunted by Atlanteans. There’s still conflict for Arthur, but now it’s directed at Atlantis and explains Arthur’s indifference to ruling. That’s far more useful for the plot than Arthur being angry at his mother.
Sometimes, when a movie isn’t working, rather than try to retcon or double-back on the story, it’s best to just ignore things outright. There are plenty of times when Aquaman gets bogged down in exposition, but thankfully it just moves past Justice League rather than trying to explain it.