Mri shows minor tonsillar herniation through foramen magnum causing crowding, max depth 5mm, no syringomyelia and hydrocephalus. what does it mean?

Depends: This is called the Chiari Malformation. Most are present from birth but can be acquired in later life. Many never produce symptoms but they classically produce headaches, problems with balance, nausea, etc. Usual treatments involve symptom management. Less often they must create more room at the base of the skull. The depth+ symptoms are often used to infer eventual need for surgery.