While a schema defines which fields a flash card can specify, a query defines which fields are displayed to you during learning as well as the field you have to enter and which fields to display afterwards.
Consider the following flash card:
You're now free to learn exactly what you want. Let's say you want to learn the capital, it makes sense to only display country and prompt for capital. If, maybe, you wanted to learn the native name of the capital you could instead prompt for capital-native. Similarly, you could learn which language is spoken or what currency is used in the country.
Note: You can also display images or play a sound as the display part of the query.