JSON schema validation is a powerful tool in MongoDB that allows developers to lock down their document data model, introducing concrete milestones and ensuring no unintended changes or unexpected data. The validation process occurs during document updates and inserts, and can be applied both during collection creation and on existing documents. By defining fields and field types, values, minimum and maximum items, and additional properties, developers can enforce strict standards on their data structure. This allows for rapid testing of different schema designs and eventual enforcement of standards, providing the benefits of flexibility and data validation in MongoDB's document model.