Still, try and define it in your own words. However, I can give you a good understanding of what naturalism is, so you may do this!
Naturalism is deeply based in artistic thinking, especially literature. It's the deep investigation on nature, and how everything ties into the natural world. A lot of times, it can be a philosophical way of thinking that describes how everything has a root in nature. Naturalistim followers often take a lot of time to experience nature in their daily lives, as they find it to be possibly the only true form of beauty. =)
Hopefully, this helps you! =)