Why we start the game with var correctGuess = false; thanks

Because later in the program, it will check by using an if condition to test if the player has got the right guess. If the player has the right guess, then the program will update correctGuess to true. The true value will be tested in another if condition later in the program to tell the player that they won the game if the condition is true.

At the start of the game, the player hasn't correctly guessed the number so the program predicts that to be false, and you will be instructing the program to check if the player has correctly guessed it later on in the game, so you will set it to false at the start of the game.