Sir J. C. Squire's short poem "The Discovery" describes the moment in 1492 when a Taino Indian sees Christopher Columbus's ships arriving at the island now known as San Salvador. I'll address the questions about theme and mood here. I suggest referring to the Guide to Literary Terms for advice on analyzing the poem's tone and figures of speech—it's a great resource for students and will give you an idea of what to look for within the poem.
The theme of a work of literature is its central idea or underlying message.
The theme of "The Discovery" is innocence—particularly innocence which is on the verge of being lost. The Taino man is startled by the sight of the ships, which appear on the horizon as if from nowhere and "[move] on the sea, by magic ... [without] one oar." He does not comprehend the true meaning of this event, because he has "known no change" in his life up to this point.
[He] stared, and saw, and did not understand,
Columbus's doom-burdened caravels
Slant to the shore, and all their seaman land.
The Taino man is unaware of the seismic shift that his world is about to undergo, which makes the poem rich with dramatic irony: we, the readers, know what "doom" Columbus is bringing to the New World, but the man only knows that strangers have arrived. He does not yet realize that his life has changed forever.
Mood is what makes readers feel an emotional attachment to what they are reading.
As the poem is from the Taino man's perspective, the mood shifts according to his mood. He originally "[strays] content along a sunlit beach," going happily about his business in a quiet landscape. The arrival of Columbus's ships changes the mood from contentment to surprise:
He heard a sudden strange
Commingled noise: looked up; and gasped for speech.
The "strangeness" of the sound is matched by the strangeness of the sight before him:
For in the bay, where nothing was before,
Moved on the sea, by magic, huge canoes.
Startled by the sound, the man is now alarmed by the sight, because he cannot comprehend how the ships are moving towards him. He assumes they must be propelled "by magic," and this thought frightens him so much that he completely forgets what he was originally doing on the beach and hides behind a rock to avoid being seen by the strangers:
And he, in fear, this naked man alone,
His fallen hands forgetting all their shells,
His lips gone pale, knelt low behind a stone.
The poem ends there, with the man crouching behind a rock, struggling to make sense of what is happening to his previously predictable life.