Well, clericalism in Iran means that the religious leaders, who are called clerics, play a really important role in how the country is run.
Imagine you have a big family and your dad is in charge of making all the rules and making sure everyone follows them. In Iran, it's kind of like that, but instead of a dad, there are religious leaders who make the rules and everyone is supposed to follow them. These religious leaders are very, very important in Iran - they have a lot of power and influence over people's lives.
Clerics are different from normal religious leaders, like pastors or priests you might see in church, because they have more of a political role. They might make laws or give orders to the government, and their religious beliefs can influence how the government runs the country.
So, in Iran, clericalism means that the religious leaders have a big say in how the country is run and what people are allowed to do. It's an important part of the country's culture and history, but it can also lead to conflict and disagreements between different groups of people.