Winter is supposed to be the hard part. The cold passes, the birds return, and everyone is meant to feel better. Burns opens by promising exactly that, "The winter it is past, and the summer comes at last", and then takes it back two lines later: "Now ev'ry thing is glad, while I am very sad." The whole poem lives in the gap between that sweeping "ev'ry thing" and the lone "I".
What it describes is simple. The speaker has been separated from a "true love" and cannot share in the season's cheerfulness. The poem never says why they are parted, whether by distance, death or quarrel, and "parted" is left open. That silence works in the poem's favour. The speaker doesn't explain the sorrow, only insists on it. The central idea is that the world's renewal cannot reach a heart that is missing someone, and that the gladness around the speaker only sharpens the loneliness.
The construction supports this. Each stanza alternates a long line with a short one, and each long line has a rhyme in the middle: "past" and "last", "glad" and "sad", "breer" and "clear", "blest" and "rest". These internal rhymes give the verse a rocking, dance-like lilt, the kind of bounce a tune can carry, and the title does call it a song. The cheerful rhythm sits oddly against the sad words, as though the music belongs to the summer while the speaker belongs elsewhere. The pairing of "glad" with "sad" is the sharpest case. The two words are bound together by sound within a single line, yet they describe feelings that cannot be shared. The end rhymes do something similar. "Tree" and "bee" are answered by "me", so the sound draws the speaker into the natural world even as the meaning shuts them out.
The imagery is bright and uncomplicated: small birds singing, a rose on the briar, clear running water, a linnet, a bee. Nothing in this landscape is difficult, and that is the point. The word "little" appears twice in one line, "their little loves are blest, and their little hearts at rest", and the effect is both tender and a little envious. These creatures have loves that are small, easily satisfied and untroubled. "Blest" and "at rest" carry a calm that the speaker, restless and separated, cannot reach. Even the rose is conceded rather than enjoyed. It "may have charms" for the linnet or the bee, and that grudging "may" suggests a speaker who can see the beauty but cannot feel it. The word "breer" (briar) also hints at thorns, though that is my reading of the image rather than anything the poem says outright.
The closing lines of the two stanzas are almost identical: "Since my true love is parted from me" and "But my true love is parted from me." Only the opening word changes, and it changes the logic. In the first stanza "since" gives the cause of the sadness. In the second "but" sets that fact against everything bright the speaker has just described. The poem then ends on the same fact it began with, without comfort, consolation or any hope of reunion. This is how grief behaves, going round and arriving at the same place. The plain wording helps too. "Very sad" is almost childlike, with no ornament and no elaborate figure of speech, and that makes it easy to believe.
Burns wrote many songs, and this one shows how little a song needs. Eight short lines are enough to turn an old expectation upside down: winter is meant to be the season of sorrow and summer the cure. Here the winter is over and nothing has improved. The poem suggests that feelings do not follow the calendar, and that the happiest-looking day can be the loneliest for the person who is missing someone. Its power comes from the contrast between a tune that wants to dance and a speaker who cannot.