In US politics the word caucus is used to refer to several different types of meeting, usually held by a group of people in private.
Leaders and important people in political parties may hold a caucus in order to choose and agree privately on candidates for public office. Until the early 1800s, caucuses decided who would be candidates for President. Now, they mostly choose candidates for local offices. Candidates for important positions are openly elected by party members through elections.
A caucus can also refer to a meeting of the members of each party in Congress or in a state legislature (= government) to decide what political action the party will take. In a majority party a caucus also decides which people will hold important positions, e.g. be in charge of committees. In Congress the word conference is sometimes used instead of caucus .
More generally, caucus can be used to describe any private meeting of politicians to decide something between themselves. Americans have a strong belief that political processes and institutions should be public and open, and so the word caucus , since it refers to a secret and private activity, is often used in a negative or disapproving way.