Basically, it is speaking of the future as if it had already happened, as if it was the past. The idea being that the prophet had seen into the future and was writing about it as if it was history that had already happened in the past.

Imagine someone in the late 1800s writing "and two towers of New York City were hit by two large metal birds, one to one tower and another to the other tower, and both towers collapsed". They are writing about future events as if they had already happened.