A rigid constitution is like the rules that we follow when we play a game. Just like how we can't change the rules of a game once we have started playing, a rigid constitution is a set of rules that cannot be easily changed. The rules are written down in a big book called the constitution, and everyone in the country has to follow them.
The reason why we have a rigid constitution is because it helps protect the rights of the people living in the country. If we were able to change the rules just because we felt like it, then it would be very unfair to some people who might lose their rights.
So, just like how we follow the rules of a game we're playing, we also have to follow the rules of our country's constitution. And if we want to change those rules, we have to go through a long process of asking lots of important people if it's okay to change them.