The poem plays with a word's history. "Lens" comes from the Latin name for the lentil, because a lens is shaped like the little seed. The speaker jokes that until the lentil "lent" its name, we had no word for the lens, even though sight is something we treasure, "the apple of our eyes." Now the lentil has become an unfashionable food, while the lens has taken over our imaginations.
From there the poem tumbles outward in a chain of lens-shaped things. First we look down, wondering what tiny creatures might be staring up from the soup. Then we look up, suspecting that lens-shaped clouds hide flying saucers, and finally out to a faraway galaxy shaped like a bulging disc.
The tone is light, playful and a little wry. The speaker seems amused by how one humble seed can lead us from dinner to the cosmos, and by how our habit of looking closely makes us see strange possibilities everywhere.