The title itself is a pun: "le ton beau" means "the beautiful tone" or "the sweet tone". But the word order is unusual for French; it would be more common to write "le beau ton". A French speaker hearing the title spoken would be more likely to interpret it as "le tombeau de Marot"; where "tombeau" may mean "tomb" (as per the cover picture), but also "a work of art (literature or music) done in memory and homage to a deceased person".
At the surface level, it treats with the difficulties and rewards of translating works (particularly poetry) from one human language to another. At points throughout the book are interspersed diverse translations to English of a short poem in Renaissance French, Clément Marot's "A une Damoyselle malade", which serve as reference points for Hofstadter's ideas on the subject.
However, Hofstadter's reading of the idea of "translation" goes deeper than simply that of translating between languages. Translation between frames of reference -- languages, cultures, modes of expression, or indeed between one person's thoughts and another -- becomes an element in many of the same concepts Hofstadter has addressed in prior works: reference and self-reference, structure and function, artificial intelligence, etc.
A particularly strong theme of this book, which is not present in Hofstadter's earlier works is the loss of his wife Carol (who died in Italy from a brain tumor) and who has also created one of the numerous translations of Marot's poem presented in the book. In this context, the poem dedicated to "a sick lady" gets yet another deeply tragic and personal meaning, even though the translations were started long before her illness was even known.