Bo-Katan Kryze returns in The Mandalorian season 2, but what has she been up to since viewers last saw her in Star Wars Rebels? First introduced in Star Wars: The Clone Wars, Bo-Katan is the younger sister of Mandalore's pacifist leader, Satine, who rule the planet during the Republic's final days and is later killed during the Clone Wars. Though she opposes her sister's pacifism, Bo-Katan always believes in a free Mandalore, and when Darth Maul attempts to take over the planet, it's Bo-Katan who leads the resistance against him.
The fight against Maul reunites Bo-Katan with Ahsoka Tano, whom she first met when still a member of Death Watch, the terrorist group who opposed Satine. With the help of Ahsoka and the Republic, Bo-Katan is successful in reclaiming Mandalore from Maul, but her reign as the planet's new regent is short-lived. Her refusal to acknowledge the newly formed Galactic Empire leads to her forced abdication, and Bo-Katan goes into exile for the next 20 years. It's not until Sabine Wren, a fellow Mandalorian and early Rebel fighter, rediscovers the Darksaber - an ancient lightsaber traditionally wielded by Mandalore's true ruler - and presents it to Bo-Katan, that she again takes up the fight to free her people.
That is where viewers last leave off with Bo-Katan before she reappears in The Mandalorian season 2, episode 3, "The Heiress". And while it's unknown exactly what Bo-Katan does in the intervening years, there are a few clues to what may have happened given the current state of the galaxy. For starters, Bo-Katan is among the survivors of the Great Purge - an event that saw the Empire eradicate the Mandalorians rather than maintain their rule over them. She has also lost possession of the Darksaber, something that likely occurred during the purge. And she has also reconnected with Ahsoka, sharing with Din Djarin the name of the plant on where he can find her.
The exact date for the Great Purge is unknown, but it must happen sometime after Star Wars Rebels but before Return of the Jedi. In order to conduct such a widespread massacre, the Empire would need to be at the height of its power, and that means the purge likely occurs before the destruction of the Death Star in A New Hope kicks the Galactic Civil War into high gear. This would place the Great Purge not long after Bo-Katan rallies the Mandalorians against the Empire, meaning it's possible her renewed resistance is what forces the Empire to take such drastic action. With the Rebellion growing, the Empire cannot waste resources continually subduing the population of Mandalore, and so instead, they wipe them out. This could also explain the lack of Mandalorians seen in the original Star Wars trilogy, with those few who did survive the Great Purge already in hiding during these years.
Among those survivors is Bo-Katan, but when she reappears years later during The Mandalorian season 2, she no longer has the Darksaber and is actively searching for it. This suggests that not only did she lose the weapon, and likely during the Great Purge, but she doesn't know for certain who has it. Traditionally, the Darksaber only changes hands through passing down across generations or by winning it in a duel, but in Bo-Katan's case, it's more likely the Darksaber was either lost in battle or stolen while she was captured or incapacitated. Assuming the Mandalorians didn't go down without a fight, the ensuing chaos of the Great Purge is the perfect opportunity for such a relic to go missing, only to be later discovered by someone who recognizes its worth - Moff Gideon.
In the years since the Great Purge and the Fall of the Empire, Bo-Katan may have kept a low profile but she has not stopped searching for the Darksaber. With her only lead being that the weapon was most likely confiscated by the Empire, that search would eventually lead her to the Imperial remnants still clinging to power during The Mandalorian. In searching for them, however, Bo-Katan surely traveled to the furthest reaches of the Outer Rim and possibly even into the Unknown Regions. This mysterious section of space is from where the First Order later emerge, but it's also where Sabine and Ahsoka go in search of their friend, Ezra Bridger, in the years after Return of the Jedi. It's possible, then, that while Ahsoka was searching for Ezra and Bo-Katan the Darksaber, the two again cross paths. Perhaps it was even Ahsoka, sharing some piece of information she learned on her travels, who pointed Bo-Katan in the direction of the Imperial forces under Moff Gideon's control.