Premove is a feature which allows you to make your move whilst it is your opponent's turn. As soon as your opponent moves, your interface plays the prepared move if it is a legal move. In a 1-minute game this can save you time, but it can also cost you pieces if your opponent played an unexpected move.
To set Premove on or off go to the menu above: Edit > Settings > 'Playing' from the list on the left. . Enable premoves is listed under 'Move input'. Simply check or uncheck Enable premoves and press OK.
For more information read "help premove"