Full many a gem of purest ray serene, The dark unfathom'd caves of ocean bear: Full many a flow'r is born to blush unseen, And waste its sweetness on the desert air.
Wait, why are we talking about gems and flowers now? Must be more metaphor. Let's take a closer look:
"Full many" is just an eighteenth-century phrase that means "lots of." So, lots of beautiful, pure gems are hidden away in dark caves under the ocean.
And lots of flowers come into blushing bloom without a human to see and appreciate their beauty or their sweet scent.
This stanza is about unsung heroes, like the guys buried in the churchyard without monuments or "trophies," and both the gems and the flowers are metaphors for people who do awesome stuff that doesn't get recognized.
Fun fact! These lines get quoted in Emma by Jane Austen, by the irritatingly self-important Mrs. Elton. Could be a sign that Austen, like Wordsworth, thought that Gray's poetry was too formal and stilted, since a character like Mrs. Elton is not exactly known for her good taste in literature. Of course, we love Thomas Gray, so this is one instance when we disagree with both Wordsworth and Austen!