The cytoplasm is the thick jelly that protects the inside of the cell. The comparison helps you understand the consistency of the protective gel. It compares the protective nature jello has to something that is inside of it. Imagine a marble inside a big jello piece, If you shake that jello the marble inside will stay secure and stay in place while the solid jiggly qualities of the jello protects it. The cytoplasm is the exact same thing.