The poem sets two kinds of speech against each other: the specialized language of doctors and the simple, everyday words people use among themselves. In the first stanza, expert talk drains the meaning out of "a quiet tongue," as if jargon wounds ordinary speech. The bleeding image is literal and figurative at once, and it suggests something living being hurt.
The second stanza is stranger and more unsettling. The tongue "bled red," and "we" were "glad to be rid of it." The poem seems to admit that people give up their own plain speech willingly, maybe because expert language feels safer or more authoritative. There's a quiet note of self-blame in that "we."
Then the last stanza turns and speaks up for simplicity. A plain word, spoken by a mouth, is "the most extraordinary thing." The mood is wary but tender, and the poem ends with a calm, firm belief in ordinary speech.