On-brand content
Published work that a familiar reader would attribute to its source with the logo removed. It matches the brand's stated audience, voice, visual system and point of view closely enough that recognition happens before the byline is checked, and it is judged against a written definition rather than against how it felt on the day.
On-brand content is not High-performing content.
On-brand is measured by attribution and high-performing is measured by response, and the two are independent: a post can be unmistakably yours and reach nobody, or go wide while sounding like anyone. Optimising only for the second is the most common route to a feed that performs adequately and belongs to no one in particular.
In practice
The phrase is used constantly and defined almost never, which is why "that feels a bit off-brand" is one of the least actionable sentences in marketing. A definition worth having has to be testable by somebody other than the founder. Cover the logo, the handle and the name, show the piece to somebody who has followed the brand for a month, and ask who made it. If they can tell, it is on brand. If they guess a competitor or shrug, it is not, whatever the palette says.
Two misreadings are common. The first is mistaking uniformity for coherence. Ten posts built from the same template are uniform, but a template is a shape, not a point of view, and a reader who could not name the source before will not be able to name it after. Recognition comes from the combination of what you choose to talk about, the position you take on it, and how the sentences sound, none of which a layout supplies. The second misreading is that on-brand means safe. A brand whose defining trait is a strong opinion is off brand the moment it hedges.
The decisive work happens upstream of any individual piece. Content is on brand because it inherited a definition, not because it passed a review after the fact. If the audience, the voice and the position are written down and the piece was made from them, being on brand is the ordinary outcome rather than an achievement. If they are not written down, every post is a fresh negotiation with whoever is writing it that day, which is precisely how a body of work drifts. This is the failure mode of generating post by post from a prompt: each prompt re-answers questions that should have been settled once.
For practitioners, two habits help. Keep a small reference set of five or six pieces that unmistakably sound like you, and use it as the comparison when something feels wrong; it is far easier to say what changed against a concrete example than against an adjective. And review a month at a time rather than a post at a time. Individual pieces almost always look fine in isolation, because drift is only visible in aggregate. Repic's approach is to generate carousels, posts and stories from one stored profile so that the inheritance is automatic, but the underlying discipline works with any tool, including none.