Picard: They must've done it in the past. They went back and assimilated Earth. Changed history. Beverly: But if they changed history, why are we still here? Data: The temporal wake must have somehow protected us from the changes in the time line.
Picard trying to explain how the Borg could have assimilated into the entire population comes up with the idea that they went back into history to change it. Beverly asks if they changed history, why they are still there. Data explains that the temporal wake must have protected them.