Think not of them, thou hast thy music too,-- (line 24)
The speaker advises autumn not to look back in jealously on spring's music. Spring never seems so distant as at the peak of autumn.
While barred clouds bloom the soft-dying day, And touch the stubble-plains with rosy hue; (lines 25-26)
The progression from mid-day sunlight to sunset could be a metaphor for the progression from summer to winter. That would be using one measure of time to represent another – a neat poetic trick.