A jar is about the humblest container there is, which is why the central image of "sunshine" works so well. Faced with a grey, windy day, the speaker doesn't ask for the sky to change. She asks for something kitchen-sized: a little sunshine kept close and ready to open. The poem is about wanting comfort you can carry, and about the small, physical misery of weather that seems to press on you from every side.
The eight lines fall into two matching movements. Each begins "On days like this," names a complaint, repeats the identical line "I wish I could contain a jar of sunshine," and ends by saying what the sunshine would do. There is no end rhyme, but the repeated line does the work rhyme might do. It gives the poem a lulling, chant-like quality, like something murmured to yourself on a bad day. The lowercase "on" that opens line 4, after the capital in line 1, adds to the effect. The voice sounds spoken rather than polished, as if thinking aloud.
The imagery is built from verbs. Clouds "fill my sights," a phrase that makes the sky crowd the eyes like a low ceiling. Two lines later the same verb returns: the sunshine would "fill me with it's warmth." The word moves from outside to inside, so clouds fill what she sees while sunshine would fill what she is. Sunshine is also asked to "settle on my skin," and "settle" suggests something small and soft landing and staying, like a blanket or a cat.
The wind is treated more playfully. It "tugs at my hair," which makes it sound like an insistent child, annoying rather than violent. Line 6, the longest in the poem, turns on a pair of touch words: the wind should go "from sharp and cold to smooth and relaxing." Sharp suggests something that cuts, and smooth something that strokes. The line stretches out as it describes the change, as though it were smoothing itself. "Relaxing" is a plain, almost everyday word, not a conventionally poetic one, but that plainness makes the wish sound sincere. The speaker doesn't want the wind banished. She wants sunshine to change its character, which is a gentler and more realistic kind of hope.
The poem never says the speaker is unhappy, but the phrase "days like this" implies that such days come round again, and the weather may stand for a mood. That is an inference from the wording, not something the text states. If it is right, the jar is a wish for an emotional remedy as much as a physical one.
The verb "contain" is slightly odd. A jar contains things, and you wouldn't normally say you "contain a jar." The slip is revealing, though, because the word carries a sense of capturing and controlling something that can't be controlled. Sunshine is about the least containable thing there is, and a jar has a childlike ring, the kind of container used for catching things. The wish is openly impossible, which explains why the speaker says "wish" and not "hope."
The ending breaks the pattern the poem has built. "oh I wish / for a jar of sunshine" drops the explanations and the weather and leaves only the longing. The "oh" is almost a sigh, and the line break holds "I wish" in the air for a moment before the object arrives. The poem has no final full stop, and the sentence trails off in the wish.
The poem's significance lies in this modesty. It doesn't dress a common feeling up as something grander. It finds one clear, memorable image for the way grey, blustery weather can wear a person down, and it respects the feeling by refusing a cheerful ending. The clouds don't part and the sun doesn't appear. The poem stays inside the wish, which is probably truer to how such days feel, and the jar of sunshine stays just out of reach.