A primary election is an election that narrows the field of candidates before an election for office. In other words, primary elections are one means by which a political party or a political alliance nominates candidates for an upcoming general election or by-election.

There are different types of primary elections, another is the series of presidential primary elections and caucuses held in each U.S. state and territory is part of the nominating process of United States presidential elections

What its not like

Primary election does not randomly elect candidates. They also don't use process of eliminations. Its main focus is to find the best candidate for a specific position in office. It i not the Progressive movement which is something other origins use to take the power of candidate nomination from party leaders to the people.

U.S. presidential elections easily explained (explainity® explainer video)