The scrum master role is a varied and cross sectional one, meaning that the scrum master works closely with the product owner as well as with the development team. The scrum master instructs the product owner, the development team as well as the business on the Scrum process while actively searching for ways to improve it, within the current context.
As a facilitator within the team, the scrum master schedules the various meetings that the team needs to have, ensuring that from a logistical perspective the team has all the resources that it requires. The scrum master will try and solve all of the development team's impediments, while trying to minimize the impact, or frequency, of external disruptions.
An important part of the scrum master's role is that of maintaining the scope of work on the agreed objectives of the current sprint, in other words changing direction mid course. The reason why the scrum master does this is because if a team is aware of how much planned work they can undertake within a given period of time, the scrum team can then better plan, estimate and be kept disruption free during the sprint.