Okay kiddo, so sometimes when boys grow up, they may have some health problems or cancer that can make them unable to have their own children. But scientists have figured out a way to save some of their sperm or even their testicular tissue to use later when they want to make a baby. This process of saving the testicular tissue is called cryopreservation.
First, the doctors will freeze the testicular tissue carefully so that it stays safe and doesn't get damaged. When the boy grows up and wants to have a baby, the doctors can thaw the testicular tissue and get the sperm cells out to use for in vitro fertilization which is like combining the freezer-stored sperm and the eggs from a female to make a baby.
It's almost like putting vegetables in the freezer to keep them fresh for later, but in this case, doctors are freezing a person's tissue that has the potential to make babies! So, in short, cryopreservation is a way to store a part of a person's body for later use when they want to have a family.