Pluralism is a theory of political science that is most closely connected to Robert Dahl, an American political scientist who wrote Who Governs, which is the seminal work on this theory, in 1961.
Pluralism holds that a democracy works in a way that is fair to all people because it allows all people to have a voice on issues that matter to them. Dahl argues that there is no one elite that can control the government in America. Instead, he says, there are different coalitions of people who control policy on any given issue. For example, those who are most interested in policy on guns will control the government on that issue while a different set of people controls policy on agriculture.
To Dahl, there is easy access to power in the American system and any group of interested people can be heard if they wish. They will not always win (because some other group may be stronger), but they will always ahve a chance to be heard.
So pluralism is a very optimistic view of how democratic and open the US system is.