"Words strain,
Crack and sometimes break, under the burden,
Under the tension, slip, slide, perish,
Decay with imprecision, will not stay in place,
Will not stay still."
(T.S. Eliot, "Burnt Norton")
Diction refers to the wording choice of an author, so to discuss diction, you would cite individual words the author uses along with how the words hint at the tone. For instance, if the author employs an austere tone, you might cite words such as "frown," "tightly pulled back hair," or "glare." The purpose of diction and tone is that their pairing in turn helps bring forth the author's intended message, or theme.