"What is that?"
Translation:वह क्या है?
When it means 'what', क्या always appears just before the verb in the sentence. In this case, the verb is है (corresponding to 'is', a form of होना-'to be'). So, वह क्या है?=What is that?
Note: क्या has another use where it can turn a statement into a yes/no question. In this usage, it always appears at the beginning of the sentence. For example, you have the statement 'वह बड़ा है। ' - 'That is big'. By prefixing क्या, you get 'क्या वह बड़ा है?' - 'Is that big?'.