Ever since 2011 DC Comics have been writing all of their comics under The New 52 banner. What this has entailed is that DC has wiped away their entire continuity and started afresh… kind of.
A large percentage of DC’s continuity was retained, such as Batman and Green Lantern’s histories.
However, it would seem that DC is trying to bring all of their erased history back… kind of.
In the pages of Justice League #40, writer Geoff Johns has revealed that everything that happened before the reboot is still in continuity.
However, this is only from the perspective of one character, Metron of The New Gods. As far as every other characters is concerned (with the possible exception of the rest of The New Gods), none of the prior events have happened and they cannot remember them.
There has been a lot of talk on the Internet as to whether this means DC will bring back their old continuity, but I seriously doubt that. This feels like a way to attempt to appease those who were unhappy with the reboot without having to bring back the old universe. In any case, we will have to see how the story unfolds in the pages of Justice League.