A medallion signature guarantee is like a secret code that makes sure important papers are kept safe. It's like a grown-up's secret handshake that only some people are allowed to use.
Let's say your mom and dad want to sell their house, they need to sign some papers to give to the new owners. But these papers are very important and valuable. If someone were to steal them and sign them, they could pretend to be your mom and dad and take all the money for themselves!
To keep this from happening, the person who handles these important papers will only accept signatures that have been guaranteed by a special service called a medallion. Only certain people, like banks and trust companies, are allowed to offer this service. They have been trained and have special equipment to make sure that the signatures are real and that the papers are kept safe.
Basically, a medallion signature guarantee is like having a secret code or password that lets people know your signature is real and that the papers are safe. It makes sure that only the right people can sign and take care of the important papers. And it keeps your family's money and property safe from bad guys.