The film world was introduced to Harley Quinn in Suicide Squad, a movie in which she teamed up with a bunch of other DC Comics characters to fight an even bigger bad guy. The character will next be seen in Birds of Prey, a film in which Harley will team up with a bunch of other DC characters to fight an even bigger bad guy.
While the new DC movie starring Margot Robbie may look a lot like the first DC movie starring Margot Robbie, Birds of Prey's writer says that the two films won't actually have that much in common. Christina Hodson produced the screenplay for Birds of Prey (and the Fantabulous Emancipation of One Harley Quinn) and she recently told Total Film (via @HarleyMovie News) that, while Prey does follow Suicide Squad, she wrote the movie as if it were its own thing, rather than a sequel.
Obviously, yes, it's a character we have met before, but it's really its own story, and that's kind of how I approached it. I just tried to take myself off-leash. I fell in love with this character, and then all of these other new characters. And I just created a brand new story that felt like its own thing, and that doesn't have to feel like it follows one or the other.
When last we saw Harley Quinn, she was being broken out of prison by her boyfriend the Joker. as far as we know, the clown prince won't be in Birds of Prey, and if the new movie isn't handled as a sequel, it's possible that there won't even be a mention as to why the pair are not together.
This fits pretty well with what we've been hearing as it regards what's left of the DC Cinematic Universe at this point. Both The Suicide Squad and Wonder Woman 1984, despite having previous films in their series, have both been hesitant to use the word "sequel." Birds of Prey at least has the benefit of only having one crossover character and an unrelated story, so it doesn't need to be a sequel in a strict sense, just the next adventure for Harley Quinn.
For the most part it appears that DC is leaving their cinematic universe behind. Movies like Shazam! and Aquaman, while technically being part of that universe, did not trade on it much at all. Birds of Prey would seemingly also be part of that world, but it doesn't sound like that is going to be important to the story at all.
In the end, what people are looking for in Birds of Prey is Margot Robbie playing Harley Quinn, she was the standout of Suicide Squad that most fans wanted to see again, and they will certainly get that. The rest is just icing on the cake.