The speaker leans on a gate opening into the woods. The season is winter and the atmosphere and climate are a "spectre-grey." There is a grey color, possibly to the sky and the frost itself, and it haunts (specter) the landscape. The speaker adds that the weakest (or bleakest) parts of winter (the dregs) make the day desolate as it turns to dusk. This is clearly a cold, bleak, depressing scene. The stems of the bushes are tangled like the broken strings of lyres. That simile here shows that the plant life (shrubs) is also desolate and dissonant in comparison with the mangled music of the lyre. Given this bleak external world, most people seek the comfort of their fireplaces indoors.
This poem was published in December, 1900. In the second stanza, the speaker notes that the landscape is like the corpse of the previous century. The cloudy sky is the century's tomb and the wind sounds its funereal song. This is an example of the pathetic fallacy wherein the landscape's bleak features match the speaker's bleak outlook on the death of the previous century and the bleak future.
The thrush, similarly bleak in appearance ("frail" and "gaunt") actually sings with joy amidst this barren and depressing landscape. The thrush seems to sing with hope. The speaker can not quite understand what the hope stems from. The previous century is dead and the speaker only sees a bleak future. The thrush provide at least the possibility of hope.