The main() function is the starting point of the C++ program execution. It starts by setting the current difficulty level at 2, and the maximum difficulty level at 10. It then sets up a loop which repeats as long as the current difficulty is not higher than the max difficulty. If the current difficulty gets higher than the max difficulty the loops ends, and the next statement get executed.
Within the loop, the following takes place. The function PlayGameAtDifficulty(difficulty) is executed with the current value of difficulty. Then the failbit is cleared and the buffer is discarded, which I think means that the game settings are reset. Then the current difficulty is increased by 1 level. This loop keeps repeating as long as the current difficulty is less than or equal to the maximum difficulty.
After the loop ends, the user is shown a message congratulating them, and then the main function ends with no error message.
Short summary: The main function plays the game at increasing difficulties from level 2 to 10, increasing the difficulty by 1 level between each attempt.