This whole poem is an apology for eating pilfered plums. But our speaker doesn't let us know that until the very last stanza. However, we can see hints of his guilt throughout the whole poem.
The Title: "This Is Just To Say" may seem like empty words, but it's actually the introduction to an apology. There's a somewhat awkward, halting quality in the title. In fact, it seems totally unnecessary, but it sets up the speaker's uneasy feeling about what he has done. He's trying to find the right words—to say the right thing.
Lines 5-8: In these lines, the speaker admits that he knew what he was doing was wrong when he was doing it. As we've mentioned before, the enjambment in these lines shows that the speaker is taking his time writing, perhaps because he's struggling with just how best to express his guilt.
Line 9: Now the speaker comes straight out and says that he wishes for forgiveness. We think his chances are pretty good, as it's nice of him to leave a note for such a minor crime. Although a "please" might have helped.
Lines 10-12: This is an interesting thing to include in any apology—an explanation that makes the crime seem worse. He's explaining why he ate the plums (they were yummy), but he's also really showing the person who was saving them just what they're missing. We might read this as the speaker's rubbing his crime in the other person's face, but we'll take a kinder approach. This description is probably a goodhearted effort to help the person who reads it have the vicarious experience of eating plums. At least we hope. Otherwise, how rude!