Adds extra time (in seconds) to your opponent's clock when you are playing a game. Maximum time you can add - 600 seconds (if you try to add more than 600 seconds, only 600 will be added).
In essence, using the moretime command means that you don't want your opponent to worry so much about the time remaining on his/her clock, about being flagged, and so on. It is a useful command to use when your opponent mentions a problem with transmission lags and you are using an interface with the autoflag feature on. You are under no obligation to give your opponent more time, of course.
If you need more time, you can always ask your opponent to add time to your clock - depending on the circumstances and their mood, they might grant your request. But if you find that you frequently need more time when you play, consider playing with longer time controls.