We ranging down this lower track, The path we came by, thorn and flower, Is shadow'd by the growing hour, Lest life should fail in looking back.
So be it: there no shade can last In that deep dawn behind the tomb, But clear from marge to marge shall bloom The eternal landscape of the past;
A lifelong tract of time reveal'd; The fruitful hours of still increase; Days order'd in a wealthy peace, And those five years its richest field.
O Love, thy province were not large, A bounded field, nor stretching far; Look also, Love, a brooding star, A rosy warmth from marge to marge.
Human memory is imperfect. The image of shadows created by the "growing hour" emphasizes that, eventually, when you look back you won't remember everything.
Tennyson seems to be consoled, though, that "behind the tomb" (wherever people end up after they die) there's no shade.
(Once again, light and dark are super-important here. If you've forgotten what this imagery might mean in the poem, better look at "Symbols, Imagery, Wordplay" again for quick refresher.)
Instead, the speaker imagines that in the afterlife, memory is like a continuous and eternal landscape. He continues this metaphor for the next three stanzas, developing it as an abundant landscape from edge to edge ("marge" means "edge" or "limit").
The five years that he knew Arthur is described as the "richest field," so the most filled up area of the landscape.
Even though the area wasn't large, since five years isn't that long in the grand scheme of things, it was a significant five years. The field that represents their friendship in Tennyson's imagination is looked over by a star that casts a "rosy warmth" across the entire field from edge to edge.