that's what she has to do=that's the thing (that she has to do)
what=the thing (that ...) 意为：.的事/物.本身作先行词和引导词,具有双重功效.再如：
那就是我喜欢的（东西）
That's what I like.= That's the thing (that I like)
全句意为：她肯定穿着鲜艳的红衣服出场,那是她必须做的.
相信你能理解赞同吧!