Pluralism or political pluralism refers to the idea that a free society can only remain free if people engage in politics. It is the right of citizens to take part. Hence, it is a significant notion of modern liberal democracies.
But why is it so important that people engage in politics? It means that people can form groups and parties to express their opinions. Further, that the government will not intervene in them and ban them. Hence, that more opinions can influence politics. Then, the public can discuss them.
A general premise, and a rising point in democracy, is that diversity of thought is beneficial. Thanks to it, that we can move forward. In summary, we all should take part, we all should have the freedom to express ourselves.

Behind the scenes
Maybe the most famous promoter of this idea was James Madison. In his famous Federalist No. 10, he argues that only a large variety of different groups and opinions working together can prevent the tyranny of the majority. Hence, everyone, regardless of race, sex, wealth, or religion, should have the chance to engage in politics.
In contrast, the lack of pluralism defines an authoritarian government. The total absence of other opinions is a symbol of totalitarianism. This note underlines the significance of allowing citizens to take part freely in politics. Hence, the opposite, monism, is undemocratic.