Do I use the past or present tense to answer this question: What is this poem about?" "

More important than deciding which tense to use for your essay is to pick one and stick with it throughout your entire paper. Grammatically speaking, either past or present tense would be considered correct, but switching between the two is sure to cause your teacher's red ink to flow.
In general, most writers prefer to use the present tense because present tense writing is often easier to read and can help make your statements sound more confident (always a good thing). Although the poem or novel that you're discussing may have been written a long time ago, you're really describing what the literature means to you today. This is known as the "historical present."