This is a sequel to Rebels and to an extent Clone Wars as it is a continuation of the stories of the main protagonists of those shows (Ahsoka, Sabine, Ezra, Hera), as well as the primary antagonist of Rebels (Grand Admiral Thrawn). You wouldn't start watching a show skipping seasons 1-4 so you shouldn't start this one skipping Rebels and key episodes of Clone Wars. You only have yourself to blame if you do.