WidgetKit is Apple's framework for creating widgets—small, interactive apps displayed on iPhone/iPad home screen, lock screen, and Mac desktop. Widgets use SwiftUI for UI, provide data via TimelineProvider, and refresh on a schedule controlled by the system. Widgets complement apps by surfacing glanceable information without requiring app launch. WidgetKit replaced older NotificationCenter extensions; it's simpler, faster, and more flexible. Widgets are a native iOS/macOS feature; users see them constantly. Well-designed widgets drive app engagement and retention.