"what you need to know"

题目

"what you need to know"
Most tutorials also recommend you need a basic grasp of HTML,but what you need to know you can learn on the fly,so although it is helpful,it's isn't strictly speaking necessary to get started.
______ 请问这句话,but what you need to know you can ...中what you need to know充当了句子中的什么成分,我想不通

答案

做主语啊~what you need to know （is that）you can learn on the fly,