The hiring manager is the person directly responsible for a job opening — typically the manager of the team the new hire will join. They define what the role needs, interview candidates, and make (or heavily influence) the final hiring decision.
It's important not to confuse the hiring manager with the recruiter. The recruiter sources and screens candidates and manages the process; the hiring manager owns the outcome and has the real authority. When you do outbound outreach, the hiring manager — not the recruiter — is usually the person worth reaching, because they're the decision-maker.
At small companies and startups, the hiring manager and the founder are often the same person, which is why founder-led hiring is the most directly reachable hiring situation in the market.