CloudKit is Apple's cloud database and storage service for apps on Apple platforms, together with the CloudKit framework, the code library that apps call to use it. An app stores its data in iCloud, Apple's cloud service, and the data stays in sync across iOS, iPadOS, macOS, tvOS, watchOS, visionOS and the web.
Data is organized in containers, one for each app, so the data of different apps stays separate. Inside a container, records hold the data: a record is a dictionary of key-value pairs, and records can link to related records. A private database keeps each user's data in that user's own iCloud account; a public database holds data that all users of the app can read. Fields in private databases can be encrypted in storage and in transport. The CloudKit Console, a web control panel, shows server activity, the database schema, telemetry and logs.
CloudKit is a hosted Apple service. It comes with Apple Developer Program membership, a yearly fee, which includes up to 1 PB of free storage for each app.