I believe they are talking about seeing the northern lights ><
"how still it is" refers to how life seems to be still around the observer
"the signal light at set of sun" when the sun sets you are only able to see them then
"shines palely green" the color of the northern lights
"a thrush sings" a thrush is a song bird
"other sound there's none" there is no sound around except for that of the bird
"nor traveller to be seen" no one is around aside from the observer, themselves
this is my interpretation of it at least ><
