The other replies explain it fine, but to add a little more specific detail:
Wedge Antilles was considered one of the best, if not the best pilot in the Rebel Alliance. His snow speeder wa the one that took down an AT-AT with a cable in Empire Strikes Back. He served in the Imperial Navy prior to Episode IV, but defected after being recruited by Ahsoka (according to the SW wiki). It appears this season will showcase his defection.
Grand Admiral Thrawn was the first main villain in the Expanded Universe, appearing in Timothy Zahn's trilogy of books in the 1990s. He is beloved by the fanbase for his cold ruthlessness and his mastery of strategy, which includes thoroughly researching his foes and their history in order to know everything there is to know about them. He was also prominent in the old canon because he was one of the few non-human agents of the Empire, which--in the now-uncanon EU--was portrayed as extremely xenophobic. Thrawn's ascent to Grand Admiral was thus a testament to his effectiveness as a commander, and it's clear Rebels is going for a similar take on him.
They're both very interesting characters--don't worry too much about not recognizing them, though. I've seen a bunch of people wondering who Thrawn is in the aftermath of this trailer, and Wedge was never really a main character in the movies, so most of his popularity comes from side material.
