One of the core problematics of political philosophy and social theory is the dynamic relationship between the individual and the community. Almost everyone will agree that the aim of a community is to protect and foster the rights and liberties of its individual members; almost everyone will agree that individuals have certain obligations and responsibilities towards their community as a whole. But the question of what that relationship should look like — how it works in practice — is highly contentious and endlessly disputed.
Generally speaking, 'individualism' refers to any theory that focuses on promoting or defending the rights and liberties of individuals, and minimizes the obligations and responsibilities of citizens to the community. The guiding principle of all individualist theories is that individuals should be free from constraint to pursue their interests, with the only caveats being the obstruction of the freedom of other individuals, and so individualist theories generally shy away from institutional structures in favor of voluntary (often contractual) obligations between individuals. Individualist theories can be Leftist or Rightist: from Marxism to syndicalism to Right-libertarianism to anarcho-capitalism.
By contrast, 'collectivism' generally refers to any theory that focuses on the needs and interests of the community as a whole, and in particular theories that try to structure a 'good society' within which individuals can flourish. Collectivism is almost always institutional, relying on laws, policies, governmental bodies, practices like voting and deliberation, etc, to moderate the actions of individuals in the interests of the society. Collectivist theories can also be Leftist or Rightist: from communism to socialism to aristocracy or theocracy.
The norm in most places over the last century or so has been some variation of representative democracy in the form of a republic: middle-of-the-road theorizing that (ideally) strikes a balance between collectivism and individualism. Unfortunately, representative democracies are generally erratic, unstable, and vulnerable to extremism, and often collapse in to autocratic forms of government.