The NIB file extension stands for 'NIB Interface Builder Archive'. These files are proprietary binary files used by Apple's development tools, primarily Xcode (and historically Interface Builder), to store the user interface (UI) layout and design for macOS and iOS applications. A NIB file contains serialized objects representing the visual elements of an application's interface, such as windows, views, buttons, text fields, and the connections (outlets and actions) between these interface elements and the underlying application code (usually written in Objective-C or Swift). When an application launches, the system loads these NIB files to construct the graphical user interface that the user interacts with. While modern development often favors using Storyboards (.storyboard files) or XIB files (which are XML-based representations of a single interface file), NIB files were the standard format for storing entire application interfaces in older versions of Xcode. They are fundamentally archives of compiled interface data, making them difficult to read or edit directly without the appropriate Apple development environment.