From the introduction:
102 years ago, giant monsters known as Titans appeared, and began devouring every human in their path. The humans had no way to fight them, and humanity as a species was almost wiped out. The remaining humans have since retreated behind three massive walls, Wall Maria, Wall Rose, and Wall Sheena, in order to protect themselves from the Titans. But this peaceful limbo couldn’t last. The Colossus Titan appeared out of nowhere to kick down the gate to Shiganshina District, the home of Eren Yeager, Mikasa Ackerman, and Armin Arlert, allowing other Titans to rampage through the town. This was followed by the Armored Titan crashing through Wall Maria, opening the interior of Wall Maria to Titan invasion. 20% of the remaining humans were killed in the following evacuation, famines, and population culling.