Premove -- allows you to make your next move without waiting for your opponent's response to your last one.
Smartmove -- allows you to make one-click moves in cases where the clicked piece has only one legal move.
Always promote to Queen -- Checking this box will cause all pawn promotions to be automatically a queen. This is useful for bullet players, since the time taken to choose which piece to promote to can cause instant temporal catastrophy, especially given that in regular chess underpromotions are so rare. However, note that if you play Loser's chess and certain wild variations with this box checked, you will lose far too often...