It's just a placeholder that says I don't yet know what I wanna put here but I wanna put something, let me get back to you in just a moment.

它只是一个占位符，告诉电脑,我想在这放一个东西，但是暂时还不知道放什么东西，我先打断一下。

哈佛公开课 - 计算机科学课程节选