Nothing is perfect, especially not software. Sometimes for whatever reason you may find that a running application froze and stopped responding to your clicks. This is often accompanied with a spinning beach ball on the mouse. Whether it’s a bug, or user error, sometimes you need to force quit an app.
You could click on the Apple menu on the top left corner, and select Force Quit, at which point you will be presented with a list of running apps, and select the troubled application.
But a much faster way to Force Quit an app is to right click on the icon of the app in the dock (or two finger click on trackpad). You will see a menu pop up above the icon. If you hold down the Option key, you will see the Force Quit option appear. Click on it, and you’re done.