Hajime Isayama's Attack on Titan is a Japanese anime/manga franchise that became a worldwide hit. The first season of the anime was released in 2013, and since then, fans have been waiting for Season 2. Part of its success was that the creators followed the manga closely, so that means no episode acted as a "filler".
Last year, it was announced that Season 2 would arrive this year, but it just hit a brick wall. Scheduled to be released this year, the series was set to make original content as the manga was still being written. But not any more.
According to a report by HNGN (via dailyherald), Attack on Titan Season 2 has been suspended because of that, and franchise creator, Hajime Isayama, and the director of the anime, Tetsuro Aaraki have both stated that the manga should always be four story arcs ahead of the anime series. This means original content won't be needed, giving fans the alignment between the manga and anime that fans expect. Right now, the manga is only two story arcs ahead of the anime, which means that Tetsuro Aaraki cannot complete Season 2 of the anime without creating original content.
Although this might be agonizing for fans who expected the new season to come out this year, I think it's the right move. We'd rather have an anime series purely based on the manga than have filler episodes that could ruin the momentum of the story. Hopefully, we'll get the new season by early next year. You can watch the first leaked footage from Season 2 here.