function startGame() {
    //disable the button to make it unclickable
    btnStart.disabled = "disabled";
    IsPlaying = true;
    renderScore();
    //start countDown function and call it every second
    //1000 is in millisecond = 1 second
    //timer variable stores refrence to the current setInterval
    //which will be used to clearInterval later.
    timer = setInterval(countDown, 1000);
}
Makes the game playable by setting IsPlaying flag to true.