The poem begins with the simple statement that the speaker has eaten plums that were stored in an icebox (a box or cupboard used to keep food cool; a precursor to the modern electrical refrigerator). This is a straightforward thing to say, but it's worth noting the way the speaker says it. Indeed, rather than saying, "I ate / the plums," the speaker says, "I have eaten / the plums."
The difference between these two phrases is subtle, but the speaker's use of the present perfect tense ("have eaten") instead of the past tense ("ate") makes the entire confession seem somehow more immediate, as if the act of eating the plums is still bringing itself to bear on the present. After all, the word "have" is in the present tense, making it seem as if the eating of the plums isn't completely over and done with, though readers don't yet know why this might be the case. The only thing that's clear, then, is that, for some reason, the speaker's experience of eating the plums continues to resonate in this moment, despite the fact that the speaker has already eaten them.
It's also significant that the plums were stashed in an icebox, since it's usually unnecessary to keep plums chilled. Consequently, the plums themselves come to seem especially precious, since the mere fact that they were placed in an icebox in the first place implies that whoever put them there wanted to treat them with great care.
Because "This Is Just To Say" is such a simple poem, both its sound and rhythm are particularly noticeable; there are, after all, no difficult words or hidden layers of meaning to untangle. For this reason, the poem's musicality is quite important, as it alerts readers to the nuances of the speaker's overall mood, which, at this point, seems directly tied to the simple act of eating chilled plums.
With this in mind, the pleasant sound of the first stanza suggests a sense of satisfaction on the speaker's behalf. For instance, the stanza includes a number of consonant syllables that pair nicely with one another, as the /n/ sound appears alongside the /m/ and /l/ sounds to create a pleasing form of euphony, one that is nicely accompanied by the speaker's use of sibilance in words like "plums" and "icebox":
I have eaten
that were in
The pairing of these consonant and sibilant sounds is very euphonic, creating an almost tactile sound that reflects the pleasure the speaker seems to have derived from eating the plums.
The pacing of this stanza is also important, as the short lines are all enjambed with one another, thereby flowing from one line to the next in a seamless, effortless way. Of course, this could make the poem sound rushed, but the enjambment between the lines actually slows readers down, as is the case in line 3, when readers most likely experience a momentary feeling of suspension as they wait for the speaker to clarify what, exactly, the plums were kept in:
that were in
This use of enjambment ultimately creates a relaxed, unbothered sound. This laid back tone aligns with the poem's use of what William Carlos Williams called the "variable foot," which can (in this case) be interpreted as an alternative word for free verse, meaning that there is no metrical consistency from line to line (though Williams would argue that this lack of consistency doesn't mean the lines are totally disordered or random).
Similarly, there is no overwhelmingly apparent rhyme scheme in this stanza, except for the subtle slant rhyme that appears between "eaten" in the first line and "were in" in the third line—a rhyme that, rather than establishing a pattern, simply adds to the poem's euphonic sound.