Fig is a tool that allows developers to easily create isolated development environments using Docker containers. It manages multiple related containers through a single configuration file, making it simple to start complex application stacks locally. With Fig, developers can define their applications and dependencies in Dockerfiles, combine them in a fig.yml configuration file, and then use Fig to build, start, and manage the entire stack with a single command. This improves the development workflow by making it faster and more consistent to test applications across different environments.