After you receive this pop-up message you can construct a colony-building in one of your socii provinces, essentially allowing you to recruit roman units in the city. The pop-up will periodically appear whenever new colonists are available and will only ever allow you to build one colony at a time.
So if you receive the pop-up and build in colony in - lets say - Capua, this will upgrade the city but you won't be able to build any more colonies in other cities until such a point where you receive the pop-up again.