The speaker is looking at the edge of a highway in late winter, when the road has been worn pale by months of salt. In the bright midday sun the tar almost shines, and there's a hint of beauty in that glare. But the speaker has to squint, and what comes into focus on the shoulder isn't pretty: the flattened body of an animal, and a strip of shredded tire.
The poem turns on a quiet joke and a quiet sadness. Calling the dead animal a "salt-lover" suggests it was drawn to the road for the very thing that has now left it there. The torn tire, curled like a fetus, echoes the animal in an odd way, as if the debris were also a body. The speaker doesn't comment or explain. The mood is cool, observant and a little grim, with the glittering surface giving way to what the road has left behind.