"that" 是一个常用的指代代词，它可以用来代替前面提到的事物或者人等，以避免重复提及，使语言简洁明了。具体指代用法可以分为以下几种：

1. 指代前面出现的名词或代词。例如：

- I have a cat that likes to sleep a lot.

- The book that I am reading is very interesting.

这里的"that"都指代前面出现的名词或代词。

2. 指代前面出现的从句。例如：

- She told me that she was going to the party, but I couldn't go because I was busy.

这里的"that"指代前面出现的从句"she was going to the party"。

3. 指代后面的定语从句中的先行词。例如：

- I met a man yesterday who said he knows you.

- I have a friend who lives in Paris, which is the capital of France.

这里的"that"也可以用"who"代替，都指代后面的定语从句中的先行词。

需要注意的是，有时候也可以用"which"来代替"that"，但它们在使用中有时会有区别，具体取决于具体语境和文体风格。