Scrum is a methodology that allows the team to self-organize and make changes quickly, in accordance with agile principles. Scrum master is a moderator for the agile development team. The Scrum master has significant responsibilities, such as:

Help the team reach a consensus on what to do over a period of time;
Help the team reach consensus on a daily basis;
Help the team stay focused and follow the agreed rules for everyday iterations;
Removing obstacles that hinder team progress;
Protecting the team from external distractions.