These aren't your everyday, run-of-the-mill socks gang. Their craft inspires our speaker, who spends a lot of time describing the socks in terms of light and darkness. No, it's not like someone's suddenly invented socks to go along with those kids' shoes that light up when you run in them. (They are cool, we must admit.) In the case of this poem, the light of the socks comes to represent their impact on the speaker, and the power they hold over his imagination (not to mention, his feet!).
Lines 13-15: The socks are described as "woven / with threads of / dusk." This is a really poetic description, which differs from the matter-of-fact subject of the poem. The juxtaposition of this fantastic dusk-thread with the ordinary "sheep's wool" is a great way to understand the poem, which is out to make elements of the everyday poetic.
Lines 26-33: These lines call the socks "celestial," having to do with the sky or heaven. In fact, the socks are so elevated that they give honor to the speaker's feet just by being worn. In this case, the light associated with these socks is a holy or sacred thing.
Lines 39-45: Now the socks are lit-up all the way—they're a "woven fire" and "luminous." While before the light-dark imagery referred to dusk (which is the end of the light of day) and things celestial (anything to do with the sky), this is full-on brightness. Full on! The socks have completely surpassed everything around them with their beauty, especially the speaker's feet.