Okay kiddo, so you know how we have rules at home that we have to follow? Well, the country also has rules that everyone has to follow and these rules are made by a group of special people called parliament. They help make decisions for the country.
And sometimes, when they decide on a really important rule, they write it down and make it official. This is called an "act of parliament".
For example, an act of parliament might say that it's against the law to steal something from a store or hurt someone on purpose. People who break these rules can get in trouble with the police and go to jail.
So basically, an act of parliament is a really big and important rule that the special people in charge of making decisions for the country create and put in place for everyone to follow.