The trailer above showcases the world beyond the wall, a far advanced civilization from the island our protagonists are from. But it showcases that the outside world is in the midst of a war with each other, and the Titans are serving as their weapons. The footage also shows how our protagonist will face off against civilization, as they are painted as the “island devils” by all who see them and the forces behind their oppression.
Crunchyroll sums up the entire series as such:
Known in Japan as Shingeki no Kyojin, many years ago, the last remnants of humanity were forced to retreat behind the towering walls of a fortified city to escape the massive, man-eating Titans that roamed the land outside their fortress. Only the heroic members of the Scouting Legion dared to stray beyond the safety of the walls – but even those brave warriors seldom returned alive. Those within the city clung to the illusion of a peaceful existence until the day that dream was shattered, and their slim chance at survival was reduced to one horrifying choice: kill – or be devoured!
Check out the final season of Attack on Titan when it arrives on Crunchyroll later this year.