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。