The title of this poem, "Introduction to Poetry," sounds like a listing in a course catalog. This poem, the title suggests, will be a little class, aiming to teach readers something new about how poetry works. The speaker, then, is a poetry teacher or professor—much like Billy Collins himself. This is a person who's passionate about poetry, and concerned that students learn how to read it.
To this speaker, reading poetry shouldn't be a dry, analytical process. Instead, using a vivid simile, he suggests that his students should treat a poem like a "color slide" (a little transparency used to project an image, like this).
This simile encourages the students—and, in turn, readers—to approach poetry with a sense of wonder and curiosity. The best way to read poetry, this image suggests, is to simply look at what's there: holding it up to let the "light" of one's attention shine through. By comparing a poem to a "color slide," the speaker implies that poetry reveals its beauty when readers take the time to thoughtfully observe it.
Take a look at the consonance in these first few lines:
I ask them to take a poem
and hold it up to the light
like a color slide
Here, crisp /k/ sounds play against gentle /l/ sounds, evoking what these lines describe: the image coming into focus on that metaphorical slide as the students "hold it up to the light." This will be a poem about why and how to read poetry—and these musical, meaningful sounds suggest that one good reason is pure pleasure.
The speaker's use of free verse in this poem reflects this sense of fun and delight. Besides making the poem's language sound conversational and approachable, free verse's flexibility means the speaker can let his ideas fall into innovative, playful shapes rather than marshaling them into a traditional form.