A code retreat is a 1-day intensive coding session where participants pair program through different coding challenges and practice techniques like test-driven development. The day consists of multiple coding sessions with different constraints, each followed by a retrospective. Participants work through problems like Conway's Game of Life, taking baby steps to write tests and code incrementally, and brutally refactoring code. The goal is to learn through pairing, extend comfort zones, practice techniques like TDD, and have fun experimenting with different approaches to coding.