I understood the words and the phrases, but I had to be edified by some online links, and I’ve got an advantage in that I know why Jakobson said it the way he did.
Exec summary: there is one takeaway message for poets:
The rest is of concern to linguists.
Selection and Combination
In structuralist linguistics, there are two structural mechanisms underlying how language works.
The syntagmatic relationship is about how words and phrases are combined to produce larger meanings. It’s syntax.
The paradigmatic relationship is about which words can be used in the slots of sentences. It’s the relationship between all nouns, or all verbs, or all adjectives. It’s lexicon.
Meaning in structural linguistics is tied up the paradigmatic relationship. Once you’ve worked out which words do the same syntactic job (nouns, verbs, pronouns), you can focus on the meaning differences between those words. In fact, the meaning of those words is defined by the available options in the paradigmatic relationship: dog = not a cat; me = not you.
That focusing on the meaning differences within an equivalence class (words doing the same job in a sentence) is the principle of equivalence.
Functions of language
Jakobson’s enduring contribution to linguistics is identifying the core functions of language. Communication is not the only function. Two functions that Jakobson pointed out, that needed pointing out, were the phatic function (keeping the channel open: “hello”, “how are you”, “ok?”), and the poetic function.
The poetic function is not just poetry: in fact, it’s not even just literature. A lot of humour is covered by the poetic function.
But the important thing about the poetic function is, that the form you use is a big part of the point of what you’re saying. It’s not just about the meaning of the words; it also about the fact that the words have metre, or rhyme, or punning similarities, or similar sounds. And so on.
The two axes
Remember: in structural linguistics, meaning is tied up with the choices of words: the paradigmatic relation. (The axis of selection.) If you use a choice of a different word, you’re expressing a different meaning. While there is also a component of meaning in the syntagmatic relation (how you put sentences together), it’s not felt to be as interesting: we’ve got nouns, we’ve got verbs, there’s a limited way of putting them together. (Remember, this is pre-Chomsky.)
Jakobson is a structuralist, and he wants to say that the poetic function of language cares about language form. So he says it in structuralist terms: We’ve been telling you that meaning is all about the axis of selection. But in poetic language, the syntagmatic relation (the axis of combination) is also a critical component of the meaning. The fact that you’ve put together words that rhyme, or that words that form a metre, or words that echo each other is just as important in the overall meaning as your initial choice of words (the strict meaning you intended to convey as a plain text communication).
We saw the principle of equivalence is how you work out the meaning of words: me = not you, dog = not cat. Different metres have different meanings too. So do different rhyming schemes. So there is a principle of equivalence at work in poetic structures as well. But it is a principle of equivalence that works on how words are put together, rather than just choices of words. So poetic language projects the principle of equivalence, from the axis of selection, to the axis of combination.
I have to say that, even without switching on Chomskian understandings of language, this is a specious way of thinking about language: different sentence structures also generate different meanings out of the combination of meaningful words, and there’s nothing intrinsically poetic about that. Semantics is propositional, not just lexical, and rhetorical, not just propositional.
But structuralist linguistics was the last time literature scholars and linguists were on speaking terms. So it was an important message for literature scholars to take in from structuralist linguistics, that poetic language is all about how you put words together, and that how you put words together separates poetic language from normal language.