In most countries with parliamentary republics (as opposed to presidential ones), the Prime Minister is the head of government. The president may formally shake hands with other heads of state and give speeches on national holidays, but the PM is the one with executive powers. They choose their cabinet and govern the country. The PM may also hold other roles or posts, such as First Lord of the Treasury and Minister for Civil Service in the UK.
The PM has a large office in the Cabinet Secretariat complex, usually referred to as South Block. It is surrounded by other ministries, such as the Ministry of Defence and the Ministry of Foreign Affairs. The office has three rungs of security, provided by the CRPF and BSF, and is staffed by senior civil servants.
A PM must be able to command the support of their party, and ideally a majority of members of parliament. If they fail to do so, their ability to make political decisions and govern effectively will be significantly diminished. This is easier with a larger majority, but even with a minority government the PM must keep up enough party unity to avoid being challenged to a leadership contest and resigning.
The position of PM was largely created in Britain in the 18th century when monarchs began to leave meetings of their ministers and other important policy decisions to the Prime Minister. The Prime Minister is not necessarily appointed for a fixed term and in some countries (such as the United Kingdom) can be replaced with a new leader of the same political party after a general election.