Garrett is reading his favorite poem. he loves the poem because it has several levels of meaning, both literal and symbolic. which literary device is the poet using?
understatement
allegory
hyperbole
irony

The poet is using an allegory.

An understatement is when you present something as less important as it really is.
A hyperbole is an over exaggeration. 
Irony is like sarcasm where you say something but mean the opposite, or when you expect something and the complete opposite happens.