The title of this poem seems like it could have been the first line. When you read the poem aloud, the title and the first line flow together nicely.
Some people say that this poem is supposed to be a "found" poem, meaning that it could have been an actual note someone left on the refrigerator, cleaned up and broken into lines. The title gives us that impression: "this is just to say" is something that a note could start with naturally. But we wouldn't call this a poem if it was your average note left on the refrigerator, and the phrase "This Is Just To Say," has more layers than it might seem to.
At first, the title "This Is Just To Say" seems to suggest that this poem has no hidden meanings—it's just to say something, for the sake of saying it. It would make sense that the speaker just wanted to say he ate the plums, and ask for forgiveness. But the idea that you could write a poem just to say what the poem says, with no subtleties or metaphors to be discovered, could make some stodgy poetry scholars roll over in their graves.
So, of course, poetry scholars have found all kinds of deep things in this poem. They've compared it to Adam and Eve, they've found sexual themes, marital strife, marital bliss. This poem, according to scholars, could have many hidden meanings.
To find different, hidden meanings in this poem would assume that the title is ironic—it's telling us one thing, but really means another. We may not be meant to believe the title at all. Perhaps, the poem hints, nothing is ever "just to say"—there are emotions and actions behind every word that is said.
But, as with much in poetry, the meaning of the title, and the poem, is up to the reader. Do you think it's "just to say," or something more?