"Literature" can be any written work but usually would refer to a group of writings that fit together in some way. This could mean a collection of books written by the same author, a set of books about the same subject, a series describing a particular event or topic.
The "nature of literature" could be interpreted to have several different meanings. It might be referring to literature that is written in a similar style - work that is all written with a futuristic, science fiction kind of setting and action. It could be that some literature is written with a common sentiment or mood - literature written from the Romantic period, for example, or as Gothic literature. You could substitute the word "type" or "kind" for the word "nature", if that helps you understand the meaning of the phrase.