Stanza 5 Summary

Get out the microscope, because we’re going through this poem line-by-line.

For the heart whose woes are legion
'Tis a peaceful, soothing region—

Turns out that coming to this creepy world actually makes the speaker feel good. He's got lots of troubles weighing him down (or, in fancy Poe speak, his "woes are legion"). Because of that, he thinks this place is "peaceful" and "soothing."
Maybe that's because the world is so bad that he'd go anywhere to escape it, or maybe it's because he gets to see his dead loved ones.

For the spirit that walks in shadow
'Tis— oh, 'tis an Eldorado!

These lines pretty much rephrase the two that came before them.
If you are so sad that your "spirit walks in shadow," then the creepy world of Dream-Land can feel like Eldorado. That's a mythical city of gold, a paradise on earth, where you could be rich, happy, etc.
Even though most people wouldn't want to hang out in the world that Poe is describing, apparently it's just great for some particularly depressed types.
This is a big shift in the poem, a surprise twist on its gloomy subject matter.

But the traveller, travelling through it,
May not— dare not openly view it!

Still, even, if a dreamer is happy in Dream-Land, it isn't his home. He will always be limited to "traveling through it."
Even more importantly, he can't ("may not") even look at it with fully opened eyes ("openly").

Never its mysteries are exposed
To the weak human eye unclosed;

The full truth, the actual appearance of Dream-Land would be too much for the "weak human eye." The "mysteries" of the place are kept hidden, apparently to protect the human traveler.

So wills its King, who hath forbid
The uplifting of the fringed lid;

It seems that this mysterious land has a king, who makes all the rules. He's the one who says that travelers can't open their eyes (or "the fringed lid" in poetry-speak) to really see what Dream-Land looks like.
Poe doesn't give us many hints about who this king is. Maybe it's the Eidolon who "reigns" on his "black throne" (line 4), or maybe it's God. On the other hand, maybe he's just another mystery, one of the many things in Dream-Land we're not allowed to really see.

And thus the sad Soul that here passes
Beholds it but through darkened glasses.

In any case, a "sad soul" like our speaker can only look at a dim version of this mysterious land. He can only see it "through darkened glasses."
We think there's a Biblical reference there (see "Allusions"), but the main point is that there's always going to be something inaccessible about this place.
This obviously makes the speaker sad – he can get a glimpse of his paradise, his "Eldorado," but he's not allowed to really look at it. That's how it goes with dreams, right? Even when they're great, you can't stay.