Yup. that's how you ask What is it?. nan/nani is what desu is a polite particle, it makes the sentence sound polite ka is the particle that lets you know it is a question, like if it was a question mark (?) I think the way you wrote it is the way it sounds, because the u in desu is silent For example: Q. Kore wa nan desu ka? (pointing at something) A. Kore wa mado desu (It's a window)

It is a question"What is it?"