Plato has a long and infamous beef with poetry and poets. Since so much about Plato seems kind of creative, and even literary, readers have puzzled over why Plato is so down on poetry. It's a mystery—and not one we can necessarily solve. But what we can explain are the two reasons Plato gives in the Republic for censoring and banning the poets.
For Plato, the big problem with poetry is this little thing called imitation (or, in Greek, mimesis). Imitation is just a word Plato uses to describe the way in which art imitates life: a poem describing a hero is imitating heroism, for example. Plato is not a fan of imitation in poetry for two reasons: 1) it can lead to bad habits, and 2) it takes us further, instead of closer, to the forms (check out "The Forms").
According to Plato, poetic imitation can lead to bad habits because it requires the poet (or, in the theater, an actor) to pretend to be something he isn't: an animal, a woman, a tyrant, and so on. The reader (or listener) does the same when experiencing his poetry. Plato worries that the more you pretend you're one of these things, the more you're actually going to become like them. So if you've just been cast as Voldemort in your school play... well, you might want to re-think that.
Plato's second objection is that poetry takes us further away from the truth of the forms. As we discuss in our section on "The Forms," Plato believes that these forms, and not the world around us, are the true reality. This means that the world around us is already an imitation of the forms: the particular chair I'm sitting in is an imitation of the perfect form of The Chair.
So that means that a nice poem about a chair is, you guessed it... an imitation of an imitation. No good. Since Plato believes that understanding the forms is the highest goal of philosophy, you can start to see why this is such an issue. Poetry leads us in the wrong direction, away from the truth and closer to mere shadows.
Now, one thing Plato doesn't talk about is what happens when poetry isn't imitative. Like, what if a poem isn't trying to imitate anything? What if it's actually trying to get us closer to the true form of a chair, for example? Wouldn't that complicate things, like, a lot? Plato doesn't talk about this, but it's important to keep it in mind before you go shredding your volume of Emily Dickinson.