Starting off with a tough question. Of course, you can Google “what is a Scrum Master”, that’s the first thing I did when I heard about the role, but the role means you wear many hats and have to be very flexible.
First and foremost, you are a servant leader, this means that you need to ensure that your team (or squad) has access to the right support, the right coaching and is protected from external interference when it does not add value. Each SM, to some extent, needs to agree with the team exactly what it is that they need to perform at their very best, this in turn will evolve exactly what a Scrum Master is for that team.
Secondly you need to get your team to a point that you are no longer needed for the day to day. You should always be working on ways to inspect, adapt and improve the processes of a team to the point where they naturally do this with, or without you. To give it some context, google the term “shuhari”, a good scrum master should be looking for ways not only to make themselves a master of scrum, but their whole team.