Objects in this world are imitating and mimicking the Forms in the permanent world. Take, for example, the Form of the triangle. The Form of it is absolutely perfect and most true and real. Any triangle that anyone draws here in this world, no matter how precise it is, is still not as precise as the one true triangle in the realm of the Forms, because it is only mimicking the one true triangle in the realm of Forms. That's why everything in this world is not as true as the Forms in the permanent world.
Objects here can become rusted, broken, chipped - they change.
The Forms never change, they are always and eternally perfect.