That's a pretty broad statement, but I'll tell you what I think literature is.
Literature is a written picture of human nature. Its characters are a reflection of people we know as well as ourselves: what they think and do and feel and say; how they react to the things that happen to and around them; how they interact with the pople in their lives...we recognize these things in them because they are us.
Literature is an opportunity to look at our own lives from a distance and through the lense of other peoples' lives and experiences. We are able to reflect on their decisions and reactions and then apply what we see to how we live our lives.
Literature is both an escape from and enhancement of reality, giving readers a chance to go places, meet people, and experience things totally out of their realm of experience.
Literature provides an opportunity to examine the lives and actions of others and understand that human nature is the one constant in history. The earliest works of literature deal with the same issues of pride and greed and selfishness and sacrifice (and more) we see in both life and literature today.
Literature is a reflection of the time and place and people in which it was written. To that extent, literature is a fictional history of mankind.
Literature allows us to enjoy the beauty of human creativity. The words, the ideas, the characters, the settings, the plots, the language, the styles....all reflect the gift of imagination and creativity which only humans have.
Whether or not this reflect the "nature" of literature, I don't know. This does capture, for me, both the beauty and practicality of one of my passions. Hope it helps.