MALE STUDENT: And then back to water again when it falls as rain or snow…as precipitation.It’s constantly being recycled, through evaporation and condensation.

FEMALE PROFESSOR: That’s it, basically. Umm … so exactly what is it you don’t understand?

MALE STUDENT: OK. I guess what I’m really confused about is how the topography of the land —the mountains and valleys and stuff—affect precipitation.

what is it you don’t understand 还原成陈述句是 It's what you don’t understand = It's something which you don’t understand

然后针对something进行提问，变成what is it you don’t understand，请问老师，which哪去了？或者如何处理对融合性关系代词what的提问？