What is a Scrum Master anyway?

Scrum is hard. Well, the idea is simple enough: Make a plan, execute the plan, and inspect and adapt as you go. However, executing Scrum well is a significant challenge for many teams and organizations. While there are a number of factors that may influence whether or not a team is ultimately successful with Scrum, the effectiveness of the team’s Scrum Master stands out as perhaps the most critical. So, that said, what does a Scrum Master do that is so valuable?