This will cause the game to return to the previous turn.

Note that I generally recommend not doing this. The main reason for this is the fact that players have already seen the current turn, and so they know what would happen given the set of moves they used last time. Hence, consciously or not, they are very likely to use this information to change their actions next time they submit a turn. Even assuming none of the players' actions are affected, there will always be a suspicion in the mind of the other players if battles turn out significantly differently. Therefore just in the interest of having the game feel completely fair, I think rollbacks are a bad idea.

A further reason is a difficulty with game file confusion. Imagine turn 10 has just come out, and someone starts to play the turn. Then the game is rolled back to turn 9, and processed again to make a new turn 10. Unless players delete the 2h file they produced from the original turn 10, Dominions will not know to ignore it and may get seriously confused. This can cause a lot of madness, e.g. soldiers becoming commanders and gaining magic paths, and so on. So if you do do a rollback, make sure to remind players to delete their 2h files.

Having said that, there are situations where rollbacks are called for, which is why I've included the option here.

Read this next bit carefully. I seem to be having difficulties explaining it coherently:

Note that once the game is rolled back, it will appear that the game is waiting for 2h files from all players. However, the 2h files from last time the turn was played will in fact be there, and will be used if the game hosts without them being overwritten. This can be useful; often, a game is rehosted in order for only one or two players to retake their turns, while everyone else is expected to make no changes. The game admin can therefore wait till just those players have submitted, and then force hosting. None of the other players will stale (assuming they actually submitted a 2h file last time the turn was played).

I hope all that makes sense!