Fixed pattern “×什么（啊）”
In Chinese the pattern “×什么（啊）” is used to brush aside someone's worry or doubt, or to stress how things actually are.
The pattern often carries a tone of:
objection;
reassurance;
“there's no need for that”.
Basic Structure
Verb / Adjective + 什么（啊）
In English, depending on the context:
What are you … for?
What do you mean, …?
Why would you …?
Examples
紧张什么啊，我们都喜欢听你唱歌，你要相信自己
Structure
紧张 + 什么啊
be nervous + what for?
紧张 — to be nervous
什么啊 — what are you … for?
我们都喜欢听你唱歌 — we all love hearing you sing
你要相信自己 — you should believe in yourself
Here 紧张什么啊 is used to take the edge off the listener's worry and reassure them.
胖什么啊，你看起来不胖也不瘦
Structure
胖 + 什么啊
fat + what do you mean?
胖 — fat
什么啊 — what do you mean, …?
你看起来 — you look
不胖也不瘦 — neither fat nor thin
Here 胖什么啊 pushes back against the listener's “I've got fat”.
急什么啊，马上就到了
Structure
急 + 什么啊
to rush / to fret + what for?
急 — to be in a rush / to fret
什么啊 — what … for?
马上 — any moment now
就到了 — we'll be there
Here 急什么啊 is used to calm the listener down.
Quick Rule
Verb / Adjective + 什么（啊）
Meaning: to push back against the listener's worry, doubt or opinion, or to reassure them.
Examples:
紧张什么啊？
胖什么啊？
急什么啊？