In Rogue One, Rebel leader Mon Mothma notes that Bail Organa has a "Jedi friend," who she wants Organa to send for upon her own request. This Jedi is none other than Obi-Wan Kenobi, the Jedi Master who trained Anakin Skywalker and is himself one of the very cruxes of the prequel trilogy.
Just before Bail Organa is about to return to Alderaan, Mon Mothma asks for him to find Obi-Wan Kenobi and persuade him to join the Rebel cause. Although Organa notes that he cannot personally go to find Obi-Wan, he will send someone that he would trust with his own life to accomplish the task. This person turns out to be Princess Leia Organa, Bail's adopted daughter and one of the children of Anakin Skywalker.
So how do Bail Organa and Obi-Wan Kenobi know each other? By the time period of Rogue One, the relationship between Bail Organa and Obi-Wan Kenobi is tethered by Leia Organa. After Padmé Amidala passed away during childbirth in Revenge of the Sith and Anakin Skywalker fell to the dark side, Obi-Wan Kenobi was left to find appropriate homes for their newborn twins - Luke and Leia. Ultimately, Obi-Wan gives Leia to Bail Organa, who then raises her as if she were his own daughter.
However, Obi-Wan and Bail Organa knew each other long before the events in Revenge of the Sith. The two first met during the Clone Wars, when Obi-Wan was serving as a general and Bail worked within the Senate on the Loyalist Committee. Obi-Wan (along with Anakin) even came to Bail Organa's aid on the planet Christophsis after Bail's relief forces were trapped by General Whorm Loathsom's assault. The bond formed during this traumatic event only solidified even more as the Clone Wars raged on and tensions within the Republic reached a fever pitch.
In Revenge of the Sith, Bail formed a strong alliance with Padmé Amidala, which ultimately brought him and Obi-Wan even closer together. Ultimately, though, the Galactic Empire prevailed over the Republic, and whatever hope the Jedi Order had of preserving the light side all but fizzled out. In the wake of Empire-organized Order 66, Bail Organa is the one who came to Yoda and Obi-Wan's rescue and escorted them back to Coruscant to warn any remaining Jedi to stay hidden. As such, he was one of the few people in the entire galaxy who knew where Yoda and Obi-Wan were after the fall of the Republic and the rise of the Empire. Essentially, these events set up what happens in A New Hope, where Leia (and subsequently, Luke) ends up searching for Obi-Wan.
Though Mon Mothma's reference to Bail Organa's "Jedi friend" in Rogue One might be as brief as any other throwaway line, it's actually a moment that's loaded with several years of history. Without the essential partnership between Organa and Kenobi, the action of the original Star Wars trilogy would not have been able to exist.
