This entire poem centers on these plums—they were saved, then they were eaten, leaving the person who saved them with no plums for breakfast. So sad, the premature death of these plums. They were, after all, delicious, sweet and cold—and one person had to go and hog them all. Even though the word "plums" is only used once in the poem, every single line of this poem refers to them, so let's take a closer look.
Lines 1-2: The split between these first two lines, which introduce the eating of the plums, is called enjambment—in fact, just about every line in this poem has some enjambment going on. Enjambment is when a thought is split by a line break, and when it's used here, it gives the reader a heads up that each little line, or segment of thought, is important, and carefully written (and connected to all the other little segments of thought).
Lines 3-4: These lines, which also use enjambment, help complete the image of these plums. They were in the icebox, or refrigerator, so we can imagine that they're cold, and maybe the morning is hot, so those plums were extra tasty. They better have been.
Lines 5-8: These lines, what do you know, show us even more enjambment. And they also start to give us the feeling that these lines are so broken up and short because the speaker feels guilty that he's eaten the plums someone else was saving. Or at the very least he's having trouble finding his words.
Line 9: So, we know the speaker has done something wrong here, but still, considering the crime—which was minor, only eating some plums—it's pretty nice of him to ask for forgiveness.
Lines 10-12: These lines show us, perhaps, why the speaker considered his crime to be so major. These plums, as he shows us through this tasty imagery, were awesome. And the person who was saving them will get zilch. Not cool dude.