The narrow definitionThe most narrow definition is that it's the one person on a single Scrum team making sure the Scrum Guide is being followed and the team successfully delivers on their Sprint Plan. Most people work in organizations that think of this narrow definition and also behave likewise.
For people looking to move into such a role, the best starting point is reading the Scrum Guide, getting a few books (such as Geoff Watts: Scrum Mastery) and a certificate or two (PSM-I from scrum.org or CSM from Scrum Alliance come to mind) then start job hunting and learning on the job.
Sticking with the narrow definition is not a very fulfilling job, as systemic impediments never get addressed.
The broad definitionThe most broad definition is that it's a person coaching an organization on becoming more agile, challenging existing paradigms and opening the way to more flexibility, and this means individuals, teams and managers on every level - from intern all the way to senior executive.
PeopleWhen it comes to being a Scrum Master in a broader sense, then Scrum is maybe 5% of the picture. In fact, Scrum itself will become less and less important and - just like the Agile Manifesto states - people and their interactions will become prevalent.
Agile FrameworksThe first thing you will need to explore is the other agile frameworks, starting with the team-level ones such as Extreme Programming and Kanban - and getting yourself familiar with the organization-level ones such as SAFe (Scaled Agile Framework), Nexus, LeSS (Large-Scale Scrum), DAD (Disciplined Agile) and S@S (Scrum at Scale). Of course, that doesn't mean you will be using all of these - but you should have a sufficient understanding to comprehend where they are useful and what their limitations are.
ManagementHaving a bag of agile theory under your belt, you need to move into organizational design and management by reading publications from authors like Drucker, Deming, Crosby, Taguchi, Taylor etc - and learning to compare and recognize their models.
PsychologyYou will also want to explore the field of psychology to understand how and why people tick the way they do. That's a vast field, where you might want to reach into behaviouristics and conditioning (Pavlov, Skinner), emotions (Ekman), basic needs (Maslov), negotiation models (Ury), transaction models (Berns), habits (Duhigg) and many others.
PhilosophyTo round this off, you may want to gain a deeper understanding of philosophy, which also has a lot to offer, with the most notable things you may want to get being logic (assumptions, fallacies, mental models), axiology and epistymology.
It's not very useful to do all of this upfront, but continuing to learn while you're on the job and working to continuously increase your sphere of influence within the organization.
SummaryThe Scrum Master is a very deep role which, when taken with a deep, serious learning attitude, has a lot to offer and provides a tremendous field for growth. If you put your heart into it, you will reach tremendous awareness of and influence on your surroundings.
Don't let people tell you that a Scrum Master is "just a team role". Great Scrum Masters can make a massive impact on their organizations.