In the poem's first lines, the speaker describes the way objects appear in the headlights of his car, as he drives home. He says,

I drive through a village at night, the houses rise up
in the glare of my headlights—they're awake, want to drink.

Here, through personification, the houses are given a kind of life: they can be awake (or not, I suppose), and they can want and drink as well. It is also possible that the speaker is comparing the houses to something else, like animals at night, without actually naming the figurative object; their description makes me think of deer, who appear, suddenly, seemingly out of nowhere, when the light from our headlights bursts upon them. Read this way, the houses are compared, via metaphor, to such animals.

The speaker also describes the appearance of sleeping people:

some can sleep peacefully, others had drawn features
as if training hard for eternity.

Some, evidently, are troubled while some are not. While there are people who are able to rest without care, some cling to sleep and seem to have to work at it, as though they were training for something. The speaker employs a simile to make this comparison.

Back outside, the speaker describes the trees as having

. . . a theatrical color, like firelight.
How distinct each leaf! They follow me home.

Another simile describes the headlights shining on the leaves of the trees. They appear to be the color of firelight, and they acquire an incredible amount of detail and distinctness, so much so that the speaker continues to think of them for the rest of the way home.

When the speaker, himself, finally gets home and can go to sleep, his mind is full of strange images, perhaps inspired by those he has seen on his drive home. Then, in the final lines, he says,

Into the slit between wakefulness and dream
a large letter tries to push itself in vain.

Thus, it feels to the speaker as though more images try to push themselves, of their own volition, into his brain as he falls asleep. A few things are going on here: first, the speaker uses a metaphor to compare his brain to a letter box into which mail can be put; second, this same metaphor compares the images and thoughts to a letter; third, this letter is personified as being able to push itself, as though it has purpose and intention.