"what"修饰的是 名词性从句。名词性从句在句子中起名词的作用，可以作主语、宾语、表语等。当"what"引导名词性从句时，它通常在从句中充当主语、宾语或表语等成分。例如：

1. What you said is true.

2. What in the world did they think they were doing？

3. What is beautiful is not always good.

4. I don't know what he wants.

在这些例句中，"what"引导的从句分别作为主语、宾语和表语。