For most of its length, the speaker seems to be in a prison cell: cold, dark, and silent, straining to catch any sound. A faint whisper in the distance gives them a little hope, but it fades, and the speaker stops trying to speak. The repeated "pray" in the middle builds a sense of desperation, moving from a voice to a face to any human contact at all, and finally to "anything but this."
The last line changes everything. The speaker is not behind real bars but "walking about the streets." The confinement is loneliness, which can feel just as total in a crowd as in a cell. Judging by that twist, the bars are a metaphor for isolation and disconnection.
The mood is tense, bleak, and quietly aching. Short lines and the pounding heart make the speaker's fear and longing feel immediate, while the ending leaves a lingering sadness: being surrounded by people is not the same as being reached by them.