Specifically, Orwell is using a metaphor to describe the effect of the British Empire. Wood is passing -- the speaker sees it but does not realize at first that the wood is being carried by old women. When he finally does see them, he realizes how old and crushed they are beneath the weight of the wood.
The Empire is much the same. It passes -- it moves along -- but as it does so, it crushes beneath it the oppressed, whose lives make it possible for the Empire to run smoothly.
