This is a great question. Plato has written a lot on the topic literature and so many thing can be stated. So, I will have to be selective. Here are two points.
First, Plato believed that literature, especially poetry was mimetic. This basically meant that poetry was an imitation of reality. Hence, we can say that literature is removed from reality. Moreover, Plato believed that often times what literature and poetry presented was also immoral. So, it would cause people to imitate things that were harmful to society.
Second, Plato also believed that literature is not based on dialectics. This is basically saying that literature does not allow a person to get to the true nature of things. Within Plato's worldview, we can say that literature does not get you to the forms, the ultimate basis for reality. The only thing that gets you to the forms is philosophy. Through philosophy, you will be able to get to the essence of goodness, beauty, and the like.