请问,有what 引导的是什么从句?如下:what really scares me is (that) I just don’t know what to expectwhat引导的是什么从句?

问题描述:

请问,有what 引导的是什么从句?如下:
what really scares me is (that) I just don’t know what to expect
what引导的是什么从句?