The speaker is responding to a request: someone has asked for their heart, and the speaker hands it over. What starts as a loving gesture turns bitter in the last line. The heart is "a dead thing," so it's useless, and the speaker says so with a shrug.
The opening "Oh!" sounds surprised, almost caught off guard, as if the speaker never expected anyone to want their heart. Then comes the offer, "Here it is," flat and casual. There's no drama in the tone, which makes the final question hit harder. The speaker seems to suggest that whatever feeling used to live in that heart is gone, perhaps worn out by hurt or disappointment, though the poem doesn't say why.
The mood is quiet, numb, and a little sad. There's also a hint of sarcasm: the speaker may be testing the other person, or warning them that they're asking for something empty. Either way, the poem leaves us with the sense of someone who has stopped expecting love to mean much.