The Scrum Master is the person in charge of making Scrum work within a team: they help the team apply the framework, run the events and remove obstacles that slow delivery. They are neither a project manager nor a line manager: their authority comes from method, not title.
In practice, they facilitate sprint planning, the daily, the review and the retrospective, shield the team from outside interruptions, help the Product Owner keep a clear backlog and support the organization in its shift to agile.
The role is found mostly in large companies and organizations structuring several agile teams. In smaller companies, it is often taken on by an Engineering Manager or a Tech Lead.
Why it matters when hiring
The Scrum Master differs from the Agile Coach, who works across several teams or the whole organization, and from the project manager, who manages deadlines and budget. A startup rarely hires one before it has several teams. The strong signal on a resume: concrete outcomes (delivery times, quality, team dynamics) rather than a list of certifications.