A petition is when a lot of people ask people in charge to do something. For example, it might be a petition asking the Prime Minister to change a law.
The person who makes the petition tries to get other people to sign it, if they agree with what it says. If enough people sign, then this can make the people in charge listen to what you are saying.
You can start your own petition or sign someone else’s petition if you agree with what they are saying.
There are websites that you can use to help you create your petition and to tell people about it. You can find out more at www.change.org.