The Clone Wars was abruptly cancelled whilst production on Season 6 was underway, leaving that series to air separately as a 13-part series on Netflix. Producer Dave Filoni was allowed to release animatics and materials detailing plans for the rest of the season, including a story arc that would reunite wrongfully-disgraced, ex-Jedi Ahsoka Tano with her former master Anakin Skywalker as they launched a military campaign to liberate Mandalore from ex-Sith apprentice Maul. This would lead directly into the events of Episode III.
Lucasfilm clearly hated leaving unfinished business behind, so this story is now being completed with a new Clone Wars mini-series that will air in 2019, with the original voice actors returning. More interestingly, it looks like this mini-series will be one of the launch shows for Disney's new adult-oriented streaming service in late 2019. Jon Favreau's live-action Star Wars TV series and a new Marvel Cinematic Universe show will join it on the service, along with a formidable amount of Disney content (and, when the deal goes through, 20th Century Fox's utterly vast backlog of shows and movies that aren't licensed elsewhere).
This will fill a hole in the storyline that spanned not just the six seasons of The Clone Wars but the four seasons of recently-concluded sequel series Star Wars: Rebels, and leave the decks clear for the next animated series, Star Wars: Resistance, which will take place in the era of the new films and focus on Poe Dameron and his X-wing squadron.