A persona is a synthetic portrait of a product's typical user – their goals, needs, frustrations and context – built from research and used by the team as a shared reference point for design decisions.
A persona turns an anonymous "target group 25–45" into a concrete figure: "Marta, marketing manager at a manufacturing company, needs board-ready reports within an hour and hates tools that require training". That concreteness disciplines discussions: instead of "I like it", the question becomes "would Marta understand this?".
The condition for personas to make sense: they must come from research, not the team's imagination. Personas invented at a desk entrench stereotypes instead of testing them.
In product practice 2–4 personas usually suffice. More dilutes focus – a product "for everyone" is for no one.