Imagine you have a special stamp that you use to mark something as really important to you. This stamp is called a sigillum dei (pronounced: sig-ill-um day). It's kind of like a secret code that only you and certain other people know about.
This special stamp has lots of designs on it that mean different things. Some parts of the design might represent God or other powerful forces that you believe influence the world. Others might represent different parts of the world or the universe.
When you use this stamp, you're saying that whatever you're marking is really special to you and you believe that the power of the things the designs represent will help protect it. Some people who believe in sigillum dei even think that the stamp can help them communicate with spirits or other entities.
So, in short, sigillum dei is like a special stamp with designs on it that represents important things to you, and using it is a way for you to protect and communicate with those things.