Three short lines hold a quiet surprise. The speaker arrives early at a chapel, before anyone else, and notices the dust. Nothing much is happening, and yet the title, "Discovery," tells us something is being found. The last line makes the leap: the dust is "as poor as a new continent."
That comparison is odd, and it's what makes the poem work. A new continent sounds grand, but "poor" suggests bare, unclaimed land with nothing yet built on it. The dust in the empty chapel looks the same way: thin, untouched, waiting. Judging by the imagery, the speaker feels a small thrill of seeing an ordinary thing as strange and new, as if stepping onto unmapped ground.
The mood is hushed, patient and a little wry. The poem suggests that discovery doesn't need a voyage. Sometimes it only needs an early morning, an empty room, and close attention to what's right in front of you.