A poem's tone is the mood or attitude that resonates with its audience after the piece has been read. This is comparable to the "tone" one takes when speaking with other individuals; tone is the emotion inherent in the words being said, whether by a boss, significant other or parent. Is the speaker angry, sad, confused, excited, etc.? When it comes to poetry, many ways exist for how to develop a poem's tone.
Diction represents word choice, and some writers use what might be referred to as "elevated" diction, synonymous with "scholarly" or "brainy" language. This kind of diction often makes use of long or Latinate words -- "clamant" instead of "loud," or "pulchritude" instead of "beauty." Because elevated diction causes a poem's words to become less accessible to readers, the poem's tone can be received as cold, condescending or calculating. However, when a poem's uses a poet's natural, everyday speaking voice, the tone becomes warmer, even familiar.
Point of View
A poem written in the first person -- what "I" did, what "I" saw -- can provide a tone of intimacy, as the events within the piece are being related firsthand with insight into the speaker's mind. When in the second person -- "you" did this, "you" saw that -- the tone often ends up seeming confrontational, direct, even commanding. Finally, in a third-person perspective -- what "we" did, what "we" saw -- the tone provided is often one of inclusion and unity.
If you imagine a line break in a poem as a brief pause, you can "hear" the pacing of the speaker's voice. Short lines that produce many pauses can offer a tone of uncertainty, a need for control, or a wary sort of progression. They can also add a lightness and simplicity of tone, as short lines are easily "swallowed" by the reader. In turn, longer lines might create a tone of urgency similar to what is heard when individuals have difficulty slowing their speech because they are excited or panicked.
Imagery affects a poem's tone through the use of detail. Are images presented gently, with sparse and quiet specifics, or are they given stanza upon stanza of simile and focus? Images that are quiet and delicate, like "a teacup in dim afternoon light," will be read as having a tone much the same. The reverse is also true -- colorful, lively images will be read as having colorful, lively tones.