There's a lady who's sure all that glitters is gold
And she's buying a stairway to heaven

There is a lady who worships money and material things, who plans to buy her way into Heaven.

When she gets there she knows, if the stores are all closed
With a word she can get what she came for

The lady "knows" (incorrectly) that even if the stores in Heaven are closed, she can get what she came for just by waving some money. Note that her idea of Heaven is a shopping mall - a delightful dig at money-obsessed cultures.

There's a sign on the wall but she wants to be sure
'Cause you know sometimes words have two meanings

Once she has gotten to the end of her stairway, she sees a sign in her (shopping mall) Heaven. That sign is likely "We're Closed", which has the double meaning of being a standard shopping mall sign and of the woman's close-mindedness. She wants to be sure because she has come a long way to find that Heaven was closed to her.

In a tree by the brook, there's a songbird who sings
Sometimes all of our thoughts are misgiven

I parsed the second line as, "Sometimes we all have thoughts that are misgiven" (as opposed to, "Sometimes all of us have thoughts that are misgiven simultaneously"). The bird is singing a song of sympathy, that it can happen to the best of us. The lady who tried to buy a stairway to Heaven was not a bad person; just misguided.

There's a feeling I get when I look to the west
And my spirit is crying for leaving

Looking "to the west" means looking into the future. His spirit "crying for leaving" suggests embarking on a quest from a place of comfort (I suspect this line was inspired by a scene of Frodo at Tom Bombadil's house before departing again on his quest). Alternative interpretation: "crying for leaving" suggests a forboding about the prospect of his and/or humanity's mortality.

In my thoughts I have seen rings of smoke through the trees
And the voices of those who stand looking

In his thoughts, he has seen a community of spirits co-existing out in the woods. The emotions he sees in these spirits are a mixture of trepidation and hope (from the spirits who stand looking), and acceptence of bittersweet realities (from the spirits making smoke rings in the woods).

And it's whispered that soon if we all call the tune
Then the piper will lead us to reason

The community of spirits is whispering (with cautious optimism) that if we choose to seek it, "the piper" will lead us on a path of reason and truth. This is a prophecy of the spirits. The piper represents a god-like figure, likely inspired by the character Tom Bombadil from Lord of the Rings.

And a new day will dawn for those who stand long
And the forests will echo with laughter

The prophecy continues: for those who can rough it out in the meantime, the path of the piper will one day lead to good places. "And the forests will echo with laughter": when this day arrives, the spirits will rejoice. The cautious optimism and partial resignation described earlier will have all but vanished.

If there's a bustle in your hedgerow, don't be alarmed now
It's just a spring clean for the May queen

Don't be alarmed if you start encountering the spirits in your day-to-day life. They are friendly and are just doing their thing. (The May queen is probably just one example of many that might have been chosen for inclusion here. Alternatively: the May queen was inspired by Goldberry in Lord of the Rings, who represents Mother Nature.)

Yes, there are two paths you can go by, but in the long run
There's still time to change the road you're on

The two paths are toward Heaven and Hell, as they are envisioned in various potential utopian and dystopian societies. "There's still time to change the road you're on": this cuts both ways, so it is both cautionary and hopeful.

Your head is humming and it won't go, in case you don't know
The piper's calling you to join him

Your head is humming the tune of the piper (and in case you don't know, it won't be going away). You are being called by the piper to join him on the path of reason.

Dear lady, can you hear the wind blow and did you know
Your stairway lies on the whispering wind?

To the lady trying to buy the stairway: the real stairway to Heaven is in "the whispering wind" (in which the piper's tune can be heard; perhaps a trust in one's own intuitions and experiences about what is true and good).

And as we wind on down the road
Our shadows taller than our soul

As we live out our lives, the effect of our wake grows continually larger. It grows to the point that it exceeds the influence of our soul. This is about the compounding effects of karma, and why it is so important to start "joining the piper" sooner rather than later.

There walks a lady we all know
Who shines white light and wants to show
How everything still turns to gold

This is probably in reference to another spirit, who wants to show people that the real path to Heaven still leads to everything you could possibly want, even if what you want is material wealth. That is to say, in the long run we can be far richer (by any metric) by following a path of reason and truth.

And if you listen very hard
The tune will come to you at last

If you listen for the tune of the piper, it will come to you.

When all are one and one is all, yeah
To be a rock and not to roll

The path of the piper ultimately leads to Heaven, where we are at one with the universe. The first line reminds me of the vision Simon and Garfunkel saw in "the naked light" (in The Sound of Silence). "To be a rock and not to roll" means to be strong and true, and not to roll (ie, onto the path that leads downward).