I don't know what it is, I don't know what it's doing, etc. The second adjective clause is the modifying, means it is not necessary but we put it in to give a little bit of extra information about the noun.
我不知道它是什么， 我不知道它在做什么，等等。 第二个形容词从句是修饰语，意思是它不是必需的， 但我们把它放在里面是为了提供一些关于名词的额外信息。