Equal parts diplomat and advocate, civil servant and CEO, the Secretary-General is a symbol of United Nations ideals and a spokesperson for the interests of the world's peoples, in particular the poor and vulnerable among them. The current Secretary-General, and the ninth occupant of the post, is Mr. António Guterres of Portugal, who took office on 1 January 2017.
The Charter describes the Secretary-General as "chief administrative officer" of the Organization, who shall act in that capacity and perform "such other functions as are entrusted" to them by the Security Council, General Assembly, Economic and Social Council and other United Nations organs. The Charter also empowers the Secretary-General to bring to the attention of the Security Council any matter which in their opinion may threaten the maintenance of international peace and security. These guidelines both define the powers of the office and grant it considerable scope for action.
One of the most vital roles played by the Secretary-General is the use of their "good offices" -- steps taken publicly and in private, drawing upon their independence,
impartiality and integrity, to prevent international disputes from arising, escalating or spreading.
(sources: https://www.un.org/sg/en/content/the-role-of-the-secretary-general; https://www.un.org/sg/en/appointment.shtml (shortened))