You get older, you just want to simplify... (when you're not trying to escape). I'm thinking about the "less is more" theory of poetry.
The person standing in front of other people, offering a kind of song... & yet even simpler, more stripped-down. Sometimes "through clenched teeth" (as Montale, I think, put it).
A spoken song, a kind of modulated speaking-chant. It's only words. It's not "an experiment". It's not high-falutin' bells & whistles. It's not "conceptual". It's not a "movement". It's not a speech, it's not a play (though it can be like a soliloquy). It's not a comedy routine. It's not a joke.
Everything is concentrated in the flow of speech, the diction, the statement, the image.
All this seems wonderfully figured in Mandelstam's comparison of poetry to a modest, unremarkable gray pebble - which contains, hidden within, a "terrifying density".
I used to think by this he meant all the baroque architecture of allusion and echoing of past poets & poems, the dense subtlety. That may have been part of it. But I think he was also trying simply to characterize the medium of poetry in general : how (unlike more complex media, like music, painting, architecture) poetry rests on this razor-thin, almost-insensible impression of verbal sound/meaning. Poetry inhabits that very thin ledge between the material (of art) & the intellectual (of thought).