"Where is the book?"
Pretty sure the duolingo notes talk about nouns and their cases. Would recommend reading some of those.
The nominative noun case tends to be the subject of the sentence, the 'thing' doing the action or being described.
E.g. John is going to school.
Mary is tall.
The vocative case is for when something or someone is directly addressed.
E.g. John, where are you going?
The genitive case describes another noun, usually see it early to show possession, but can be used in different ways.
E.g. John's book
The book of John.
A crown of gold.
Singular means that there is only one of the person/thing. A book. The book.
Plural means there are more than one (at least two). Books, The books.