It said something to the effect of all my cities build queues being completed for the first item in queue, next research would be completed, and +10% population in all my cities. It seemed pretty overpowered, but then I've only ever had one of these, so they must be pretty rare.
I did get one of my cities to go up a level that round, but other than that I didn't notice any of this happening. The event stayed around in my quest list until the end of the game... but that was only like 20 seasons away.