A noun, as a basic definition, is a word that designates a person, place, or thing. Nouns can be simple and generic, like book, or they can be more descriptive, like romance, mystery, or thriller. The more descriptive your nouns, the more vivid your writing will be.
For example, consider the noun dog. Dog is a very nondescript noun that only conveys a very basic meaning. Chihuahua, terrier, and dachshund are more descriptive, and therefore effective, choices. Such words paint a recognizable picture in the reader's mind, making your writing seem more lively and specific. Try to choose nouns that are colorful and descriptive, rather than generic.