The wind was a torrent of darkness among the gusty trees,
The speaker of this poem opens strong, with three big metaphors all in a row.
First he compares the wind to a "torrent of darkness." We imagine that "torrent" being like a black river, rushing and swirling along.
The moon was a ghostly galleon tossed upon cloudy seas,
The next big metaphor compares the moon to a "ghostly galleon." A galleon is a big old ship, the kind that would have carried Spanish gold across the seas. So the moon is like a ship sailing through the sky.
You see the mood he's aiming for here? This is a spooky nighttime scene, a good setting for a sad, scary story.
The road was a ribbon of moonlight over the purple moor,
Now he introduces the road, which is a major location in this poem. On this dark, windy night, it shines bright and white, a thin strip of light like a "ribbon of moonlight."
The bright road runs through a purple "moor" (those are the open, grassy fields that you find a lot in England). They aren't really purple of course, but the night and the moonlight must be making them look that way.
And the highwayman came riding-- Riding--riding-- The highwayman came riding, up to the old inn-door.
Here comes our main guy – the highwayman. All these lines tell us is that he's riding up to the old door of an inn.
Of course they do it with a little more style than that. They especially emphasize the repetitive rhythm of his "riding--riding--riding." You'll notice that this is a pretty simple story, but the speaker works hard to make it gripping for the reader.