Metal is Apple's API (application programming interface) for graphics and general computation on the GPU, the graphics processing unit: a chip that runs many small calculations in parallel. Metal comes with its own shading language, in which developers write shaders, small programs that run on the GPU. It works on iPhone, iPad, Mac, Apple TV and Apple Vision Pro.
An app calls the Metal framework to send work to the GPU directly, with low overhead, to render complex 3D scenes or to run computational tasks in parallel, including machine-learning inference. Related frameworks add to it: Metal Performance Shaders supplies a large library of optimized compute and rendering shaders, MetalFX upscales rendered frames, and higher-level frameworks such as RealityKit, SpriteKit and Core Image run on top of Metal. PyTorch can use Metal to train models on a Mac. The Metal debugger, a performance HUD and validation layers help developers find errors and slow steps. Metal 4 needs an A14 Bionic chip or later, or a Mac with Apple silicon.
Apple lists no separate price for Metal. Publishing an app on the App Store requires a paid, yearly Apple Developer Program membership.