Lines 1-4 establish the poem's setting: a freshly built playground, or "new recreation ground." The time of year is early autumn, as "Summer is fading" (line 1) and "leaves" are "fall[ing]"—not thickly, just "in ones and twos"—from surrounding "trees" (lines 2-3).
Along with the title, "Afternoons," the autumn imagery is symbolic. Afternoon is when the sun is past its high point; autumn is when the year is past its midpoint and its warmest season. Accordingly, afternoon and autumn are often used to represent other kinds of decline, such as the decline of youth, strength, or beauty. Note, too, how the speaker says the line "Summer is fading" rather than "Summer is ending." This might evoke other common kinds of fading (the fading of energy or memory, for example).
Put all this together, and the poem seems to be establishing a mood of wistfulness, melancholy, or nostalgia. Though it's set on a brand-new playground, it seems a bit somber and concerned with the passage of time.
The lack of a rhyme scheme or meter establishes that the poem is written in free verse. The enjambments at the ends of lines 2 and 3 make the lines seem to tumble down the page with little punctuation to slow their fall:
The leaves fall in ones and twos
From trees bordering
The new recreation ground.
This effect subtly mirrors the falling of the leaves. The repetitions of consonance and assonance (the /f/ sounds in "fading," "fall," and "From"; the /-ing/ suffix of "fading" and "bordering"; the /ee/ vowel in "trees" and "leaves") also echo the repetitive leaf-fall, giving this first stanza a hushed, gentle music.