Warning: Contains SPOILERS for The Mandalorian season 2, episode 3, "Chapter 11 - The Heiress."
Bo-Katan Kryze appears in The Mandalorian season 2, episode 3, "Chapter 11 - The Heiress" and, while not made explicit in the installment itself, she is the title character. The Mandalorian season 2 has continued the adventures of Din Djarin and Baby Yoda, as the former attempts to reunite the latter with others of his kind. To do so, he's tracking down others of his kind, fellow Mandalorians, who can point him towards the Jedi.
As with season 1, The Mandalorian season 2 includes new cast and characters within each episode, and so far the title of each one has referred to a particular newcomer. The Mandalorian season 2, episode 1, was titled "The Marshal" after Cobb Vanth, the Marshal of Mos Pelgo, while episode 2 was then called "The Passenger" in honor of Frog Lady, who journeyed with Din aboard the Razor Crest. However, in episode 3, "The Heiress", the titular character isn't quite so obvious.
Played by Katee Sackhoff, The Mandalorian season 2, episode 3 introduces Bo-Katan to a live-action Star Wars setting, having previously appeared in both The Clone Wars and Star Wars Rebels (where she was also voiced by Sackhoff). As Bo-Katan explains here, she is of clan Kryze and was born on Mandalore, making her a much truer Mandalorian than Din Djarin is, but what's not spoken is that she is, in her mind at least, the heir to the darksaber and believes herself to be the rightful ruler of Mandalore. So, though tempting it may be to believe that the "Heiress" is the tadpole baby of Frog Man and Frog Lady (and to be fair, there's little to counter that notion), the title does refer to Bo-Katan.
House Kryze was a powerful family on Mandalore, with Bo-Katan's sister, Satine, becoming its Duchess. Bo-Katan, who rejected the pacifist ways her sister believed in, later helped overthrow her rule and established the Death Watch and its leader, Pre Vizsla, as the ruling power on Mandalore - until Vizsla himself was defeated by Maul. Bo-Katan and her Nite Owls refused to recognize Maul's authority as a non-Mandalorian, and a civil war ensued. Eventually, Maul was defeated, and Bo-Katan was named the Lady of House Kryze and Regent of Mandalore. That's a position she wouldn't hold for long since shortly afterward Order 66 was executed and, following that, Bo-Katan refused to serve under the Empire, but it's enough to begin to hint at the title "The Heiress."
Having fought against the Imperial occupation of Mandalore, Bo-Katan was recognized by Clan Wren - including Sabine - as the person worthy of wielding the darksaber and holding the title of Mand'alor, the leader of the Mandalorians. While she initially resisted this honor, she would later come to accept it, taking on the mantle and the darksaber that came with it. While there's much room for debate and interpretation as to who the darksaber truly belongs to, since it's generally passed down either through generations or when taken from the defeated enemy who used it, the choice of Clan Wren was enough for most to accept Bo-Katan as the rightful leader.
With the Great Purge having since happened, with many Mandalorians being wiped from existence and Moff Gideon claiming the darksaber for himself, the Bo-Katan's return as the "Heiress" who the weapon belongs to is crucial to her story. As The Mandalorian season 2, episode 3 demonstrates, she is hell-bent on retrieving the darksaber from Gideon, believing it hers by right and that she is the one who can help lead the Mandalorians once again, something that would be much easier to manage with the legendary black lightsaber in her possession.