Someone is asking the speaker for help, or for love, and the speaker is willing to give it even though they are running on empty. The poem's title, "rock bottom," tells us where they are standing: at the lowest point, with nothing left in reserve. The central image is a heart broken into pieces. The speaker is ready to lend "a piece" of it, but all that remains are "crumbs."
The odd, stretched-out layout matters. "here / i / am" breaks into single words, so the speaker seems to appear slowly, almost hesitantly, as if pausing before each step. Then the last line runs long and flat, ending on "crumbs" with a quiet thud.
The mood is tender but heavy. There's generosity here, but also exhaustion and a bit of sad humor. The word "lend" suggests the speaker hopes to get something back, which makes the offer feel even more vulnerable.