The speaker imagines dying tomorrow and knows exactly how they would spend the last day: with one particular person. The poem is really about love and absence, and death is simply the setting that makes the feeling clear. If time were running out, nothing else would matter as much as having this person close.
The middle lines turn sadness into something physical. It "seeps into the cracks" and "slithers" through the muscles until the bones shake. Those verbs make the longing feel slow, creeping and almost like an illness, something that takes over the body when the loved one is away. Then the poem returns to the deathbed, and the speaker asks for one thing: this person's presence, to "make the void disappear."
The mood is quiet, vulnerable and a little desperate. The short, plain lines and the closing "only you" make the poem sound like a whispered confession, with nothing else wanted and nothing held back.