A coup d'état is like a big game of pretend where a group of people decide they don't like who is in charge of their country and they want to be in charge instead.
So, they make a plan to take over the government all at once, like in a surprise attack. They might do things like take control of important buildings, like the president's office or the police headquarters, and try to get as much power as they can.
When they are successful, they become the new leaders of the country, and the old leaders might be arrested, exiled or even killed.
But it's important to remember that a coup d'état is not a good thing. It often leads to lots of violence, chaos, and instability and can take away people's rights and freedoms. So it's always better to find peaceful ways to solve problems and disagreements.