Metal is a low-overhead graphics application programming interface (API) developed by Apple Inc. Metal combines functions similar to OpenGL and OpenCL under one API. It is intended to improve performance by offering low-level access to the GPU hardware for apps on iOS, macOS, and tvOS. It can be compared to low-level APIs on other platforms such as Vulkan and DirectX 12.
Metal is an object-oriented API that can be invoked using the Swift or Objective-C programming languages. Full-blown GPU execution is controlled via the Metal Shading Language. According to Apple promotional materials: "MSL [Metal Shading Language] is a single, unified language that allows tighter integration between the graphics and compute programs. Since MSL is C++-based, you will find it familiar and easy to use."
Metal was first included with iOS 8 and OS X El Capitan. It requires a mobile device with at least an Apple A7 processor, or a Mac with GPU hardware from 2012 or later. With the release of macOS Mojave, a Metal-compliant GPU became mandatory.