What is this? 뭐에요?

As we learned in [is/am/are], when there is a book, you can say

책(book)이에요.

But, when you don't know what it is, you need to ask what it is. In Korean, when you make a question, word order of the sentence doesn't change. Normally, the last part of the verb changes, for example, "다녀오세요." becomes "다녀오셨어요?", however, not every verb change. 이에요/예요 is one of verbs that do not change to make a question. You simply add a question mark or raise the end, if you're speaking. 

뭐(what)에요?

"뭐에요?" is a simplified sentence where the subject is omitted. If you want to specify, you can bring it back. To ask "what is this?", you can say

이(this)거(thing) 뭐에요?

Therefore, when there is a book,

A: 이거 뭐에요?
B: 책이에요.

a desk:

A: 이거 뭐에요?
B: 책상이에요.

a dining table:

A: 이거 뭐에요?
B: 식탁이에요.

Quizlet: https://quizlet.com/_1vzpqr