I don't have any symptoms, but an MRI shows a chiari malformation.  what does this mean?

Then leave it alone. Various cerebellar malformations fron i to IV or v depending extent , to severe symptoms in type i just cerebellar tonsils herniate to spinal canal as shown ( most likely you have this ) most of the time asymptomatic other have symptoms need complex corrective procedures, .
Three types. An asymptomatic chiari malformation shows a low situated cerebellum, it is asymptomatic and referred to as type i. Types ii and iii show the same, but a progressively worsening impingement on the foramen magnum and the occurrence of cervical spine impingement-giving rise to neck and shoulder numbness.