UIKit is an application development environment and graphical user interface toolkit from Apple Inc. used to build apps for the iOS, iPadOS, and tvOS operating systems.
[1] UIKit provides an abstraction layer of iOS, the operating system for the iPhone, iPod Touch, and iPad.
UIKit is insipired by the AppKit found in the macOS Cocoa API toolset and, like it, is primarily written in the Objective-C language.
UIKit allows the use of hardware and features that are not found in macOS computers and are thus unique to the iOS range of devices.
iOS, watchOS, and tvOS technologies can be seen as a set of layers, with UIKit at the highest level and the core operating system / kernel at the bottom.