A shared plan for information
An online shop might define a customer record with an ID, an email address, and a sign-up date. Its schema says which details are required, what format they use, and how an order links to a customer.
Such rules can describe database records, API messages, or analytics events.
Changes affect other systems
Changing a schema can affect existing records and connected apps. If the shop changes the format of its sign-up dates, old records still use the earlier format. Teams may need a data migration to update those records and adjust apps that use them. Even if a system does not enforce every rule, the shared description tells people what to expect.