2609 is a possible future for humanity in which Observers reside. By this year, the world has become so degraded that they can not live there anymore. As a result, Observers are sent back in time to study their evolution and lay the groundwork for a full-scale invasion meant to take the planet back.
In Relationship to Timelines
2609 doesn't fit on any timeline because it hasn't physically happened yet. It would be improper to label it on the Original Timeline because that timeline ends with the world ending around 2026. This future would never be able to unfold, which is the reason why the creation of an Alternate Timeline was needed. 2609 can not be labeled on the Alternate Timeline either, however, because it existed before the creation of the Alternate Timeline. Logically, the "original" timeline that the Observers sought to restore after September's mistake was one in which Peter Bishop died and never existed as an adult in the Prime Universe.
Within the frame of the series after Walter's crossing, 2609 can best be described as one of countless possible futures, but it would not get placed on the timeline until the timeline has reached that point. 2609 is existing, presumably simultaneously with other futures, until a definitive future comes to pass. This future will never come to pass because Walter's Plan was a success.