A tone of voice is a brand's defined way of speaking and writing: word choice, sentence rhythm, level of formality and emotion. It's what lets you recognise a brand's communication even without the logo.
A visual identity defines how a brand looks; tone of voice defines how it sounds. The same message can be written like a government office, like a mate from next door, or like an experienced adviser – and the choice of style tells the customer who they're dealing with before they've read the content.
A tone of voice document usually contains: brand personality (best expressed as "we are X, but not Y"), language rules, a vocabulary of the brand's own phrases plus a banned list, and above all "before and after" examples on real copy. Without examples you're left with adjectives, and every copywriter will turn "professional yet approachable" into something different.
Increasingly, tone of voice is also written with AI tools in mind: if a brand's copy is produced with language models involved, a document in a format AI understands helps keep the style as consistent as an editor would.
For buyers: ask for a sample on your own copy. A rewritten fragment of your website will tell you more about the document than a chapter of theory.