Docker is an open-source platform that automates the deployment, scaling, and management of applications using containerization. Docker packages applications and their dependencies into lightweight, portable containers that can run consistently across various environments, from development to production. By isolating applications in containers, Docker ensures that they are not affected by differences in the host system, improving reliability and efficiency. Docker includes tools for building, sharing, and running containers, making it easier for developers to manage application lifecycles and collaborate across different stages of development.