Yes, the new show is called “Ahsoka” and most of the stuff you need to know is about Ahsoka. But since the show is going to pick up the story from the end of Rebels, with Ahsoka and Sabine going to find Ezra and Thrawn, there are a few other things to know.
Hera Syndulla is a Twi’lek Rebel leader who flies the Ghost. She was in love with a Jedi named Kanan Jarrus, who trained Ezra Bridger but who eventually perished in their fight with the Rebellion against the Imperial forces on Lothal. Kanan and Hera had a son, named Jacen, conceived before his death.
Sabine Wren is a Mandalorian artist and warrior who swore to her friend Ezra to watch over his home planet of Lothal. Which she has ever since Ezra and Thrawn disappeared.
Grand Admiral Thrawn is a brilliant military strategist—a blue-skinned Chiss, part of a civilization called the Ascendancy that exists beyond the currently-charted regions of Star Wars galaxy (to the perspective of our heroes) in the Unknown Regions. Thrawn is now the apparent heir to the Empire since Palpatine is currently “dead.” He, along with Jedi in training Ezra Bridger, disappeared during the Battle of Lothal before the events of A New Hope.