Bo-Katan Kryze (Katee Sackhoff) returned in The Mandalorian season 2 and told Din Djarin (Pedro Pascal) about Ahsoka Tano; here's how Bo-Katan is connected to the former Jedi. Although Bo-Katan just made her live-action debut, she has been part of Star Wars for years. The Mandalorian first appeared in Star Wars: The Clone Wars, returned briefly in Star Wars: Rebels, and then came back again in the final season of Clone Wars.
The Mandalorian season 2 set Din on a path to find more Mandalorians who could connect him with a Jedi, and he finally encountered some in episode 3. Bo-Katan and a few other Mandalorians helped save Din and The Child twice from angry Quarrens. But, Din has to help Bo-Katan on a mission to find out the location of a Jedi. Once he helps Bo-Katan, Koska Reeves (Mercedes Varnado/Sasha Banks), and Axe Woves (Simon Kassianides) overtake an Imperial vessel, Bo-Katan tells Din where he can find a Jedi named Ahsoka Tano in the city of Calodan on the forest moon of Corvus.
Bo-Katan and Ahsoka have a long and complicated history from the days of the Clone Wars. They first met when Bo-Katan was a member of Pre Vizla's Death Watch, a radical group of Mandalorians looking to return the culture to its previous warring ways, and leader of the Nite Owls, an all-female group of elite fighters. They set up a camp on the snow planet Carlac and were contacted by the son of a senator who Ahsoka was tasked with accompanying on this journey. Ahsoka wasn't meant to reveal her Jedi nature, but she did when Death Watch began destroying a local village. This led Death Watch to subdue her, and she was nearly executed by Pre Vizla before she escaped. Bo-Katan had to track her down again, but Ahsoka got the best of the Mandalorian warrior.
Ahsoka and Bo-Katan crossed paths again after the former had left the Jedi Order and the latter was no longer associated with Death Watch. This time, they united their forces to take on a common enemy, Darth Maul. After getting some help from Obi-Wan Kenobi and Anakin Skywalker, Bo-Katan and Ahsoka traveled to Mandalore to take the city back from Maul. The war that ensued is now known as the Siege of Mandalore. Bo-Katan and Ahsoka led the attack to liberate the planet and were successful in doing so. They parted ways with mutual respect for one another.
Although Star Wars: Rebels featured both Ahsoka and Bo-Katan, they didn't cross paths again during the four-season run. However, Bo-Katan knowing the whereabouts of Ahsoka during The Mandalorian indicates that they have kept in contact. This could point to them teaming up on other adventures throughout the years, or Bo-Katan could be in contact with fellow Mandalorian Sabine Wren, who was last seen going on a mission with Ashoka. With Ahsoka's live-action debut in The Mandalorian imminent, perhaps future episodes will provide more background on Bo-Katan and Ahsoka's continued friendship.