What we did was to add a property to the Order called IsValid

what we did 是整个句子的主语，what 是 did 的宾语。

what 引导主语从句，做从句的宾语，主语从句做整个句子的主语

就是be动词后面的，add a property to the Order called IsValid

这里的what是引导词，没有什么实际的意义，就是代表为了增加所有权而做的事情。