Smoke is about the aftermath of a breakup, told through the image of a fire that has been put out. The speaker once felt warm and alive, and that warmth came from being loved. When the other person ended things, it felt like someone blowing out a flame, leaving only smoke behind.
That smoke is the heart of the poem. It stands for what's left of the love: not enough to burn, but enough to remind the speaker of what they lost. The line about being "hot enough to be re-lit, but not hot enough to start" captures that stuck, in-between feeling, where the past still lingers but nothing can move forward.
The mood is sad and a little bitter, but it softens by the end. The speaker cries for days, yet also looks back and feels blessed to have been loved. The last line says it plainly: that love is what kept them ablaze.