Hyperbole is exaggeration. As you read the poem, while there may be something that could happen, there is usually an element of is that is grossly exaggerated and could never happen or is a claim that could never happen. For example, "I am so hungry I could eat a house." In this particular poem, "A long brown bar at the dip of the sky (where the sand or beach meets the sky) puts an arm of sand in the span of salt is hyperbole. The beach doesn't really have a an arm in the salt.