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You have a .swift file — source code for Apple's modern programming language. Swift replaced Objective-C as the primary language for building iOS, macOS, watchOS, and tvOS applications. If you're writing an iPhone app today, you're almost certainly writing Swift.
Apple introduced Swift in 2014, designed by Chris Lattner (who also created LLVM). The language takes the best ideas from Objective-C, Rust, Haskell, Python, and C#, then packages them in a syntax that's significantly more readable than its predecessor. Swift is type-safe, memory-safe (via ARC — Automatic Reference Counting), and fast — compiled Swift approaches C-level performance. SwiftUI, Apple's declarative UI framework, makes building interfaces look almost like writing pseudocode.
Xcode (free, macOS only) is the primary IDE — it's required for building and deploying to Apple devices. VS Code has Swift extensions for non-Apple platforms. Swift is open-source and runs on Linux too, but its primary home is Apple development. To get started: install Xcode from the Mac App Store, create a new project, and start writing.