Max Turns is one of the advanced game setup options; it enables you to specify the game length in turns, in advance. When you reach that point, the game's over. I tried using it after trying to play a Civ V game to completion, and failing because I got terminally bored. I now set Max Turns to about 200, and I'm happier, because the game stops before boredom sets in. I reckon that, if I'm already winning on score by turn 200, the rest of the game would be a waste of time, so I may as well declare victory and stop. And, if I'm not winning on score by turn 200, I'm not doing well, so I may as well give up. I play at Prince level because I don't like playing with handicaps.