Things kind of go from weird to weirder now. After the speaker has them holding the poem up to the light, now he wants them to press an ear against it (next time you fall asleep in class—you know, head down on the desk, drool pooling on the pages—try using this as your defense, "Billy Collins said to listen this way!").
And the speaker doesn't just say press an ear against the poem. That would be way too normal at this point. He wants them to press an ear against the poem's "hive." When we read the word hive we think bees, we think honey, we think danger, we think sweet but even more importantly we probably hear the buzzing.
It is probably this auditory (sound) trigger that Collins was shooting for. He is telling us to listen for the sounds in the poem. He has turned the buzzy, honey-drippy hive into a metaphor for sound in poetry. The qualities of sound, what it can convey (danger, anticipation, excitment), the energy and importance of it in poetry, are reflected in the qualities of the hive that the speaker wants us to press our ear against.
Read line 4 aloud a couple times and you'll find Collins has given us a good example of exactly the kind of listening he is talking about. If you listen carefully (did you reaaaaaaaalllly read it aloud? Mumbling the words while you listen to your ipod doesn't count) you'll notice some repeated "S" sounds and a prominent "V" sound that give line four a buzzing quality, "or preSS an ear againSt itS hiVe." Hear it? If not, turn down the music and try again. You can actually feel the buzzing on your lips when you speak the word "hive."