To explain what a Scrum Master is and what they do, it's necessary to start with the definition of what SCRUM is, and there's no better way than to simply translate the words available on the scrum.org website:
"Scrum is a framework within which people can address complex adaptive problems, while creatively delivering products of the highest possible value.
In short, Scrum requires a Scrum Master to promote an environment where:
A Product Owner orders the work for a complex problem.
The Scrum Team turns a selection of work into a valuable Increment.
The Scrum Team and its stakeholders inspect the results and adjust.
Repeat."
This "formula" is extremely simple but requires a real shift in thinking within the company that wants to become agile through Scrum.
Below, I will describe the parts that make up a Scrum Team and the roles assigned to them.
The Scrum Master is responsible for the proper implementation of Scrum as defined in the Scrum Guide. They achieve this by helping everyone understand the theory and practice of Scrum, both within the Scrum Team and the organization.
A Scrum Product Owner is responsible for maximizing the value of the product resulting from the work of the Scrum Team. How this is achieved can vary significantly among organizations, Scrum Teams, and individuals.
Developers are the individuals on the Scrum Team who commit to creating every aspect of a usable Increment in every Sprint. The specific skills required by developers are often extensive and vary depending on the domain of work.
Like everything else, in my opinion, the first step to becoming a true professional is to study and demonstrate that you know what a Scrum Master should do.
There is a specific path to becoming a certified Scrum Master, and you can find all the necessary information on scrum.org.
I took a paid course for the first certification and then got all the other certifications studying on the Scrum.org website, so I suggest you find an authorized entity directly on scrum.org.
I can recommend a website where you can take practice tests and prepare thoroughly for the final exam: thescrummaster.