PostEaseThe short answer
A brand voice is the consistent tone, vocabulary, and personality a business uses across its content: formal or casual, playful or direct, technical or plain-spoken. It is what makes an account's writing recognizable as theirs, independent of the logo or handle attached to it.
A real test for whether a voice exists
A useful check: could a specific post, stripped of any branding, plausibly be attributed to this account by someone familiar with it? If any generic post from any account could pass as this brand's, the voice is not distinctive enough yet to be doing real work.
How to develop one
A brand voice usually starts from a small number of deliberate choices: a handful of adjectives describing the intended tone, a short list of words or phrasings to avoid, real examples of posts that hit the mark and ones that missed. Writing these down, even briefly, gives anyone producing content something concrete to reference instead of guessing at tone each time.