It's a little puzzle piece that says when the green flag is clicked and this is kind ; of like the beginning of a program; when you double click an icon on your desktop this is how something like that is implemented; it's listening for that double click, it's listening for that green flag.

是一小个按钮，它的意思是,何时点击绿色旗帜,这就好比开始运行程序;,当你双击桌面上的图标,就开始执行操作,桌面的图标需要双击，绿色小旗也等待点击来执行命令。

哈佛公开课 - 计算机科学课程节选