return game.applyGuess(guess) how this work and guess is pass to another method ? dont understand this line .

In an application code is executed line by line so he already declared that the char that accepts the argument of guessInput which is "0" , he named that guess....so once the code in that line is executed it then moves on to the last line in the method with the return value... being that the guessInput is already assigned to the variable guess in the promptForGuess scope he can now use it any where after he declared it as long as its in that scope..... so in theory when the executed line 14 he can use it in any line after which is what he did, he called the applyGuess method from the game class and he the passed in the guess that was assigned. Hope that helps as i'm also learning and i'm not a master yet.