Okay, so a long time ago, in the early 1900s, the government in one of the states in the United States decided that all kids had to go to public school. This meant that if you were a kid, you had to go to school in a big group with lots of other kids, and the government decided what you would learn and how you would learn it.
But some parents didn't like this rule. They thought it was important to be able to choose where their kids went to school and what they learned. They wanted to be able to send their kids to private schools or religious schools where they could learn about things that were important to their family.
So the parents of the private and religious school kids got together and filed a lawsuit, which means they asked a judge to decide if the rule was fair or not. The case was called Pierce v. Society of Sisters.
The judge agreed with the parents and said that the government can't force all kids to go to public school. Parents have the right to choose where their kids go to school and what they learn. This became an important idea in America, and now parents can choose to send their kids to all different kinds of schools, not just public ones.
So basically, Pierce v. Society of Sisters is a court case that said parents can choose where their kids go to school, and they don't have to go to public school if they don't want to.