The poem pairs two uneasy images: a clock offered as a birthday kiss, and a coffin that holds only an echo. A birthday normally marks a year gained, but here the gift is a clock, a reminder that time is running out. The speaker is asked to kiss it, as if affection and dread have become the same thing.
The title, "Mortal coil," points to the idea of life as a burden we eventually shed. The poem seems to place the speaker in that moment of shedding, "buried in this coffin" with nothing but an echo for company. That detail suggests loneliness, and perhaps the sense that a life leaves only faint sound behind.
The final lines are the most fragile. Something is "so thin" that "the very breath broke," which could describe the speaker's voice, their hold on life, or the poem itself giving way. The mood is quiet, claustrophobic and unsettled. Nothing is shouted. The fear comes through how small and breakable everything feels.