"that"在英语中是一个非常常见的代词，有多种用法和意思。以下是一些常见的用法和意思：

1. 指示代词：用来指代已经提到或将要提到的事物、人或概念。
例句：I saw a dog in the park. That dog was very friendly.

2. 连词：用来引导一个从句，进一步解释或补充前面提到的内容。
例句：She said that she would come to the party.

3. 引导定语从句：用来引导一个从句，修饰前面的名词或代词。
例句：This is the book that I bought yesterday.

4. 引导结果状语从句：用来引导一个从句，表达结果或效果。
例句：He was so tired that he couldn't stay awake.

5. 引导名词性从句：用来引导一个从句，充当名词的功能。
例句：I believe that you will succeed.

6. 形容词用法：有时作为形容词，表示“那个”或“那样的”。
例句：I want to buy that red dress.

7. 代表事实或证据：用来引用或强调已知的事实或证据。
例句：It is a fact that smoking is harmful to health.

请注意，具体使用方式和意思会根据上下文而有所变化。