Dave Bonta's "Under the bridge" is a single line, and it leaves a lot unsaid. A person sits alone in the dirt, and that dirt is described as "another's bed." The phrase suggests the speaker has settled into a spot where someone else sleeps, or has slept. Maybe that person is gone for the moment, or gone for good. The title points to a place of shelter for people with nowhere else to go, though the poem never says so outright, so that reading is an inference.
What the speaker feels is left for us to work out. The word "alone" is plain and heavy, and there's a quiet sense of trespass, or of borrowed comfort. The speaker might be an observer imagining this scene, or someone sharing in it. Either way, there's no complaint, only a flat, stripped-down statement.
The mood is bare and hushed. With no punctuation and no explanation, the line feels like a glimpse of something private, and it asks us to sit with the absence of the person who isn't there.