What's a simple way to say what chiari malformation really is?

Chiari: Chiari malformation is said to exist when the lowest part of the cerebellum of the brain (called the tonsils) extend below the foramen magnum (the hole in the base of the skull through which the spinal cord descend into the spine). In most cases this is of no consequence. However, minor or major symptoms are possible. This is almost always a congenital condition (you are born with it).