Stockfish is a chess engine: a program that analyzes a chess position, evaluates it and searches for the best move. Players reach it through a separate chess app, a desktop GUI (graphical user interface) or a website, which talks to it through UCI, the Universal Chess Interface: a text protocol in which the app sends a position and a search command, and the engine answers with an evaluation and a best move.
Its evaluation uses NNUE (Efficiently Updatable Neural Networks), a small neural network trained on billions of chess positions. Its search is an alpha-beta search, a method that skips lines of moves that cannot change the result, with aggressive pruning. Every code change is tested on Fishtest, a distributed framework that plays millions of test games on computers lent by the community. Stockfish runs on Windows, macOS, Linux, Android and iOS, and it has won the Top Chess Engine Championship.
The project started in 2008 as a fork of the Glaurung engine and is now maintained by the Stockfish community. It is free and open source under the GNU General Public License version 3 (GPL v3).