It takes place before/during the Revenge of the Sith. The hologram of Padmé which helps to frame the timeline. It will be interesting to actually see how Ahsoka Tano battles with Maul playout during the siege of Mandalore. Ahsoka Tano is heavily featured in the trailer. We see her with Bo-Katan Kryze. We know that Ahsoka Tano was secretly allied with her during the clone wars. She is also the one who ends up with the Dark Saber after it is taken from Maul. Moff Gideon got it some time after that which we saw in The Mandalorian. We also get glimpses of Anakin moving to the dark side. It will be interesting how this last season will dovetail into The Mandalorian and The Kenobi series which is in the works.