The Great Truth is found in the motto which says, "We are one in all and all in one. There are no men but only the great We, One, indivisible and forever"(19). Basically, there are no individuals, only everyone. The Unmentionable Times are those days before the society was created. The Uncharted Forest is the area around the city where it is illegal to go. The Evil Ones are those who fought against the development of the current society/government as well as anyone who might revolt again. The Great Rebirth is when the new society was fully embraced and all of the rebels were either killed or converted to the philosophy. Those are the basic literal meanings, but each term also has a symbolic meaning. For example, the Uncharted Forest also represents the personal journey that each person must go through in order to discover themselves, who they are with others, and who they are as an individual. It is in the Uncharted Forest that the brotherhood does not have control over the individual; however, if the brotherhood can keep scary stories circulating about the vicious animals who live in the forest it will help them to control people within the city walls.
Anthem is Ayn Rand's paean against collectivism, and it champions Rand's philosophy -- the ultimate in individualism -- called objectivism. The Great Truth, then, in this society that pushes collectivist principles to the extreme in order to show their flaws, is that there are no individuals, only the collective. Only the collective has value. The Unmentionable Times refers to the time before the Great Rebirth, the time when individuals were privileged and the good of the collective was sacrificed to individuals' selfish desires before society realized the Great Truth of collectivism. It was during the Unmentionable Times that people knew and used the word "I" to refer to themselves instead of "we". The Evil Ones are those people who fought then or still fight against the collective and insist on the importance of the individual.