To simplify, for people who aren't familiar with the format, Simultaneous Turns means that at any point during the Turn Period (e.g. 48 hours), a team may connect to game and execute moves. Micromanage their cities, move the units, propose trades in the diplomacy screen, establish research paths, &c. At the end of the turn period - in this case 48 hours - the pitboss host server's clock will chime 'Midnight' and all of the teams' turn will be forced to end, if they have not yet clicked 'Enter' to end the turn manually. So if a team has not connected to the server and executed moves, then their units won't go anywhere; build queues will be commandeered by the game's AI engine, and no diplomatic offers will be accepted.