A speaker has spent hours trying to find the right words for someone, and the poem is what's left after that effort fails. The title, "you," in lowercase-feeling quotation marks, keeps the person unnamed and close, as if the reader is being let in on a private moment.
The central idea is simple: language isn't big enough for this feeling. The speaker points to "twenty six different letters," the whole alphabet, and says no arrangement of them could capture even "a sliver" of what they feel. That small word, sliver, is telling. The speaker isn't saying the words would be incomplete, but that they'd barely scratch the surface.
The mood is tender and a little frustrated, but mostly sincere. There's something quietly funny and touching in using words to say words are not enough. The poem's plainness helps. It doesn't try to dress the feeling up, which makes the admission feel honest, and the feeling itself, never named, seems all the larger for it.