A poem's speaker is the constructed voice doing the talking inside the poem, and it's a genuinely distinct figure from the poet who actually wrote it, even when a poem uses the first person "I" and even when that voice draws heavily on the poet's own real experience. Tone is the attitude that speaker takes toward the poem's subject, sincere, ironic, bitter, tender, and reading a poem well means actively asking who is speaking here and in what tone, rather than simply assuming the poet is addressing the reader directly and straightforwardly in their own voice.
Treating the speaker as automatically identical to the poet causes real misreadings
A poem written in the first person by a poet who has never committed a murder can still convincingly voice a murderer's own perspective, and a poem's speaker can hold views, express emotions or occupy a situation the poet themselves doesn't share at all, using the invented speaker specifically to explore that perspective from the inside rather than reporting the poet's own direct opinion. Collapsing the speaker into the poet, assuming every first-person "I" is simply the poet talking about themselves, is exactly the kind of misreading that causes readers to miss a poem's actual irony, its dramatic distance, or its deliberate inhabiting of a voice unlike the poet's own.
Tone tells you how to actually read the speaker's words, not just what they say
The same literal statement, "what a beautiful day," carries an entirely different meaning depending on whether the speaker's tone is genuinely admiring or bitterly sarcastic, and a poem signals that tone through word choice, rhythm, context and the gap, if any, between what's said and what the surrounding poem suggests is actually meant. Reading tone accurately is exactly what separates a careful reading of a poem from a surface-level one, since taking a genuinely ironic speaker's words at simple face value produces close to the opposite of what the poem is actually communicating.
What we're still unsure about
That a poem's speaker is analytically distinct from its poet, and that tone shapes how a speaker's literal words should actually be read, are well established, foundational principles of poetry criticism. What's more genuinely a matter of ongoing interpretive judgement is exactly how confidently a given reader can identify a specific poem's tone or the precise distance between speaker and poet, since that judgement depends on genuinely contestable inferences about context, irony and intention that different careful readers sometimes reach different conclusions about, and critics continue to actively debate specific poems' tone rather than that identification being a simple, mechanical exercise with one clearly correct answer.
This sits inside Voice, Tone & Speaker, one of seven topics in Poetry, one of five domains in Literature, one of seventeen subjects the app can quiz you on.