"Aspens" by Edward Thomas is a poem that, at its most simple level, is about aspen trees and how they constantly blow in the wind, no matter who is near or what is going on in the world. The meaning is that aspens are an eternal part of this world, and as everything else changes, dies or fades away, the aspens will still be there
"at the cross-roads talk[ing] together/Of rain, until their last leaves fall from the top."
Edwards strikes a rather ominious tone in the poem when he declares that
"The whisper of the aspens is not drowned,.../No ceasing, calls their ghosts from their abode."
Here he is saying that the whispering of the aspen leaves is never stopped, and it always calls ghosts out from hiding. That is an interesting and creepy thought. Edwards thinks that their noise is like "grieving", and that no matter who or what is near, the aspens will continue to grieve forever.
Edwards uses a rhyming scheme of ABAB, which means that the first and third lines, the A lines, rhyme, and the 2nd and 4th lines, the B lines rhyme. He uses personification to give the trees human-like traits. They "whisper" and "grieve". He uses imagery (the five senses) to make the aspens seem more alive, and the atmosphere more mysterious. He describes the "bare moonlight" and "thick-furred gloom," of the nighttime, the "lightless pane" of abandoned houses, the "empty lane" of deserted streets. Using imagery like that creates a more lonely and mysterious mood.
I hope that those thoughts can help to get you started on the path towards understanding the poem more fully. Good luck!