It shows the standard deviation of the return on the portfolio as a function of the expected return on the portfolio.

它是投资组合的收益标准差,关于预期收益率的函数图像。

耶鲁公开课 - 金融市场课程节选

So, the optimal thing to do if you live in a world like this n is to get n as large possible and you can reduce the standard deviation of the portfolio very much and there's no cost in terms of expected return.

如果现实中也这样简单的话,那么你就尽量增大,这样就能让投资组合的标准差,就会大大降低,从预期收益率的角度来看,这样做的成本是零。

耶鲁公开课 - 金融市场课程节选