What does prime minister of UK do?

The Prime Minister of the United Kingdom of Great Britain and Northern Ireland (PM), becomes PM when is asked by the Queen (the head of state) to form a government (the cabinet) after their partys victory in a General Election. The PM is the chairperson of the cabinet and is responsible for appointing members of the cabinet to the various ministries of state (the Home Secretary the Chancellor, the Health Secretary etc.). The Ministers and Cabinet undertake the duties of Government by leading the civil service departments for which they are responsible.

The Cabinet and its ministers are also responsible for drafting legislation which is then debated and voted on in the house of Commons so that it can become law.

The PM also answers questions in parliament twice a week. He/she also meets with the Queen (HM) once a week, where HM advises, warns and/or cautions the PM.

By convention the PM is also the leader of the party in power (except when coalition governments are formed such as that during WW2 and the current)

There are more things the PM does but the above is the core of it.