whose concord shall not fail; (line 3)
Based on this quote, Wordsworth seems to believe in some kind of enduring, absolute standard in the universe, whether or not it’s "truth." The world isn’t just chaos and anarchy; there’s an order. The Greek philosopher Plato once described it as "the harmony of the spheres."
Truth fails not; but her outward forms that bear The longest date do melt like frosty rime, (lines 7-8)
The idea of truth "failing" is a bit strange, but we’ll go with it. The quote states that truth has an "inner" and an "outer form." Sounds like the old philosophical distinction between essence and appearance (Plato again!). Appearances can change, but essence never does.
That in the morning whitened hill and plain And is no more; (lines 9-10)
You could draw from this quote the somewhat radical idea that truth takes on different appearances at different times. You can’t rely on the standard of appearances as a measure of absolute truth.