With season 3 of Star Wars Rebels barreling toward its inevitable conclusion, there's still a lot of Star Wars story to cover. The first two and a half seasons helped to establish that there are many smaller Rebel cells that occasionally coordinate or share resources, but there's no larger overarching hierarchy or chain of command, other than the fact that people seem to listen to Bail Organa.
With Rogue One: A Star Wars Story (and obviously A New Hope), we've seen that these Rebel cells eventually come together to form the much more cohesive Rebel Alliance. As Rebels approaches the timeline of the original trilogy, it only makes sense for the story of that unification be told, and it looks like at least part of that story is coming in the next episode: 'Secret Cargo'.
In the preview for 'Secret Cargo', Senator Mon Mothma has apparently just given a scathing speech, taking a stand against the Empire, hoping her efforts will serve as a lightning rod, drawing more Rebels to the cause. The only issue is that she's still a member of the Imperial Senate, as she will be until it's dissolved after the Death Star plans are stolen. As such, while she can speak out against the Emperor's tyranny, she can't participate in an open and organized rebellion.
That's where Hera and the Ghost crew come in. Mon Mothma appears to be the eponymous "secret cargo", and Hera is tasked with transporting her safely to Dantooine to participate in a summit or other secret meeting of would-be rebels to finally form the Rebel Alliance.
Outside of the momentous occasion that is Mon Mothma forming the Alliance, this clip introduces a few things that have been floating around Star Wars lore for a while. The first is Chandrila, Mon Mothma's homeworld. While Chandrila is far from new in Star Wars canon - it's actually been a big focus for much of the new books, especially the Aftermath Trilogy - this marks the first time the planet has been mentioned on screen. It even sounds like the events of this scene take place in the same system as Chandrila, meaning there's also a possibility it will also appear on screen for the first time.
The second is Dantooine. you don't have to be a hardcore fan for that reference to be familiar. Dantooine is the name of the planet Princess Leia gives Governor Tarkin in A New Hope when they press her for the location of the Rebel base. We find out later in the movie (and in the novel Lost Stars) that there were remains of a Rebel base there, but it had been abandoned for some time. Now we know why. The rebellion itself may not be new, but it sounds like Dantooine just might be the birthplace of the Rebel Alliance.
Secret Cargo appears poised to finally tie up several loose threads Star Wars fans have known about for years, but with another Maul/Kenobi battle and attack from Thrawn on the horizon, season 3 appears poised to go out with a bang.