A buyer persona is a detailed profile of the person you're trying to convert. It covers their goals, pain points, objections, decision-making process, and the language they use to describe their problems. Done well, it's the single most useful document for writing landing page copy.
Done poorly — which is most of the time — it's a fictional character sheet with a stock photo, a made-up name like "Marketing Mary," and demographic details that change nothing about your page. If your persona doesn't change at least one decision about your landing page, it's useless.
What a useful persona looks like
Skip the demographics unless they directly affect messaging. Instead, focus on three things: (1) What problem are they trying to solve right now? (2) What have they already tried? (3) What objection will stop them from converting? These three answers should drive your headline, your supporting copy, and your trust signals respectively.
The best persona research comes from actual customer interviews, support tickets, and review mining — not brainstorming sessions. Read how your customers describe their problems in their own words, then use that exact language in your headlines. The gap between how you describe your product and how customers describe their problem is where conversions go to die.