The reason why I do put them up is so that you have the opportunity if you want to see it again or if you have to miss a class.

我之所以把资料放在网上是为了,让你们可以重温或者学习错过的课程。

哈佛公开课 - 幸福课课程节选

So sometimes, if you miss a class, you can just kind of check in earlier years, like lectures,

所以有时候，如果你错过了一节课，你可以看看前几年的讲座，

开放式的课程？ - SpeakingMax英语口语达人

What this means is that if you miss a class you need to get notes, and so you should get them from a friend or from the person sitting next to you.

这也就意味着,如果你落下了一节课,你就要看笔记,你可以向朋友或者身边的同学借来看

耶鲁公开课 - 心理学导论课程节选