Every so often, the game is reset, to make room for new tasks and new players (or old players who have nowhere to go but up to the top of the Tower). And to make it important to do those little, special level 1 tasks again.
ERA refers to the following tasks:
At the start of a new era, everyone's score is reset, and all active tasks are retired. New (or not-recently-active) tasks are added to the list.
Era changes bring significant changes to the game's mechanics, updates to the super-custom SFZero software, and adjustments to way the game's meaning is articulated. Previous changes include player-submitted tasks, teams, new forms of group membership, and various changes to the voting mechanism.
There have been five eras so far: Impossible Exchange, Glasnost, Interregnum, Insatiability, and the current era, Everyday Life.