The "Play Best" button plays the best move for you in the current position. However there will be a large penalty for using the button, so be careful how often you use it. Each time you use "Play Best" there will be a certain number of moves added to your "sub-optimal" move count (used in calculating your rating if you succeed). The penalty for using "Play Best" will be a minimum of 10 moves. If you have made any previous attempts then the penalty will be the longest attempt you had made already for the current position plus an extra 5 move penalty (or 10 moves if this number is still less than 10). The purpose of "Play Best" is to provide an alternative to completely giving up, and thus to allow you a chance to learn more about a position which you may otherwise feel completely lost in.