What it means
Brand voice is the consistent character of your writing across a caption, an email, a menu and a reply to a bad review. It is defined as much by exclusions as by adjectives: the words you do not use are usually more useful than the ones you do.
Why it matters to a small business
It is the only part of a small business's marketing that a chain cannot copy, because it comes from a real person with a real point of view. It also does practical work: a voice written down as rules is the thing that stops five different people, or five different tools, producing five different businesses.
The bit people get wrong
Three friendly adjectives are not a voice, because nobody can act on them. Warm, professional and approachable describes almost every business in Britain. What is usable is specific: we say prices out loud, we never say passionate, we write like we talk to a regular, we do not use exclamation marks.