Servant-leadership is all about collaboration, trust, empathy and the usage of power ethically. Scrum master is often called as the servant-leader for the Scrum team.
As a servant-leader scrum master is responsible for:
Setting up Scrum as a servant process, not a commanding process
Guiding the Development team towards self-organization
Shielding the team from disturbance and external threats
Helping the team make visible, remove and prevent impediments
Leading the team through healthy conflict and debate
This way scrum master is both a servent as well as a leader.