Last season,
Laurel was killed by Damien Darhk with an arrow. It was an emotional death and she was gone for a while. Because of her death, Oliver began to reflect on why he even started this lifestyle. And while Oliver slowly got over it, in Season 5 he once again reflects over why he did these things; it caused John's family to be captured, Felicity's boyfriend to be killed by Oliver himself, and Curtis's boyfriend leave him because of his lifestyle. This leads Oliver and the viewers to believe that everything he does leads to pain, especially since Oliver thinks that him killing someone in season 1 is connected to Prometheus's scenario.
Yet at the end of the episode, standing before the mannequin of Black Canary that Oliver used to reminisce and mourn, is Laurel.
Earlier in the episode, Thea told him that his legacy wasn't only mistakes. But Oliver only focused on the wrongs and thinks that there can be no good. Oliver is contradicted when Laurel returns, showing something good can happen.
There is a lot of speculation about how she came back, but there are two main theories; one is that Flashpoint altered the timeline, and the other is a bit more confusing.
In the 4-night crossover, the heroes are captured in a fantasy world and have to find a portal in order to get back to the real world after an alien abduction. In this world, Laurel was alive, as a way to get Oliver to stay there. There is a possibility that Laurel followed Oliver to the Smoak building that allowed the heroes to go back to their lives. Somehow returning to the material.
It may also be that she's a ghost. However, it is unlikely. Nonetheless, Laurel's return could lead to Arrow's the main couple reuniting. The rest of the season could possibly be about Oliver and Laurel because Felicity is away and John is in jail. This would please the Comic audience of Arrow, as previously they were angry over Laurel dying and Felicity getting more screen time.
But no matter which way Laurel has returned to the Arrowverse, it's certain that there will an emotional divide between Oliver's happiness and guilt.
Massive
ARROW SPOILERS