Trojasmic is quite right that it is an end turn button, but with a twist: as long as there are still actions possible/needed to be taken, the button will tell you (or take you there). For example if you have a unit that still can move or a city that is idle, the button will take to said unit/city instead of simply ending the turn.
Trojasmic,
I though a little bit of extra clarification was in order to explain the difference between an "end turn" and a "shafer" button, and in no way intended any ill regard or diminishment toward your answer.
