Wordsworth is describing a universal experience. When we are children the world looks like a wonderful, magical place, partly because everything is so new to us. He was especially aware of this truth because he was a sensitive, poetic person. As we grow older and become preoccupied with the cares of adulthood, we lose the wonder of childhood along with the immortality we take for granted since it hasn't yet occurred to us that we are mortal. Children can be happy anywhere, doing virtually anything. They are especially happy when out in nature, such as playing at the beach, playing in a park, climbing trees--doing all the things children delight in doing. But we are all fated to become burdened with adult responsibilities, desires, fears, and worries, including financial worries. Wordsworth himself had serious financial worries because he was supporting his wife, his sister, and eventually five children. His "Ode: On Intimations of Immortality" is his most famous and most frequently anthologized poem, probably because so many people recognize its universal truths. (But it may be difficult to appreciate these truths when one is still young.) In the poem he traces the decline of youthful joy and wonder until the grown adult sees it "fade into the light of common day."
The poem also addresses the ephemeral, or short-lived, nature of things. We can focus on the stanza in which these lines appear to address that:
Ye blessèd creatures, I have heard the call
Ye to each other make; I see
The heavens laugh with you in your jubilee;
My heart is at your festival,
My head hath its coronal,
The fulness of your bliss, I feel—I feel it all.
O evil day! if I were sullen
While Earth herself is adorning,
This sweet May-morning,
And the children are culling
On every side,
In a thousand valleys far and wide,
Fresh flowers; while the sun shines warm,
And the babe leaps up on his mother's arm:—
I hear, I hear, with joy I hear!
—But there's a tree, of many, one,
A single field which I have look'd upon,
Both of them speak of something that is gone:
The pansy at my feet
Doth the same tale repeat:
Whither is fled the visionary gleam?
Where is it now, the glory and the dream?
Those who feel happiness are "blessed" and the "heavens laugh with [them] in their jubilee." Some of us can identify with this. When things are going well, it seems that the entire universe is working in our favor.
The poet muses that it would be "evil" to be "sullen" on a "May-morning" so lovely. Wordsworth uses metaphors related to timelessness—Earth, the heavens, and even the traditions of festivals and coronations—then juxtaposes them with mundane images of children picking flowers ("children are culling") and "a babe [leaping] up on his mother's arm." The flowers will eventually fade, and the "babe" and children will grow up, making this glorious morning a distant memory.
He is reminded of this when he sees a tree in a field. He recognizes this spot of earth; both are associated with another memory, perhaps of a similar day he experienced in the past: "Both of them speak of something that is gone."
He then focuses on a single "pansy at [his] feet." His choice of a pansy in this line is significant for several reasons. Firstly, it is a common flower that can grow nearly anywhere, even from pavement. It is appropriate here because all of the visions that Wordsworth describes—the children picking flowers and the babe with its mother—are common sights. Secondly, the etymology of 'pansy' comes from the French word 'pensée,' which means "thought." The flower was regarded as a symbol of remembrance, which is exactly the practice in which the narrator engages. Third, the narrator is focused on a single flower, a reflection of his own solitariness.
In the end, these images—both those that signal a moment from the past and those experiences that exist in the present—tell the "same tale": something wonderful, or at least our perception of something wonderful, has been lost. The sense of perception is emphasized in the phrase "visionary gleam," for a vision is that which we see; it is very subjective. A gleam is a small, faint light—something transient and faint, like many of our memories of moments past. A day that seemed filled with "glory," like a "dream," is now gone, but he does not know where, hence the question at the end of the stanza.