A is comprised of tall trees (generally around 80 meters tall), which have enormous trunks, large branches, and can grow larger than most Titans. Due to their enormous sizes, humans can stay atop the upper branches of the trees and remain safe from Titans, which do not possess the intelligence to climb trees. They are used by the Survey Corps for certain goals on expeditions. These forests are excellent places to use the vertical maneuvering equipment, as there are many tall tree trunks and branches to which the steel cables can secure to, improving versatility and mobility. At the same time, it is a good place for Titans due to the large tree trunks concealing them and the denseness of the forests reducing visibility of incoming Titans.
Story: During the 57th expedition to the lands beyond Wall Rose, the entirety of the Survey Corps' outer right flank was wiped out, due to the Female Titan apparently leading an army of Titans. Commander Erwin, the leader of the expedition, signaled the center column to enter the pathway to the Titan Forest, located somewhere between Wall Maria and Wall Rose. The remaining units, who were not brutally killed by the Female Titan, were ordered to station themselves on the borders of the forest and stop any Titans that attempted to enter.
Trivia: There are several of these forests in the region between Walls Maria and Rose and they originally functioned as tourist spots. According to Zeke, the Titan Forest is a rather unique place in the world.
Source: Titan Forest, available under CC BY-SA 3.0.