Telling the model to act as a particular kind of person: a careful editor, an experienced accountant, a sceptical reviewer. It shifts vocabulary, tone and what the model attends to. It does not add knowledge or ability, despite a great deal of advice implying otherwise.
Technical view
Conditions generation on a persona, measurably affecting style, verbosity and default assumptions, with modest and inconsistent effects on task accuracy. Evidence for persona-based accuracy gains is weak and does not reliably reproduce across models. It is most useful when the role genuinely disambiguates the task, such as specifying an audience, and least useful when substituted for explicit instructions about what is wanted.
Related
Part of AI Terminology Index, section: Controlling and Extending Behaviour at Run Time.