Swift is a general-purpose programming language developed by Apple for developing applications for iOS, macOS, and more. Until now, apps for these platforms could be run and developed only on the Mac.
This includes the compiler, the standard library, and the core libraries (dispatch, Foundation, XCTest). These libraries are part of what enables developers to write powerful applications with ease and without having to worry about many of the details of the underlying system.
You can read more about Swift below.