Docker is an open-source platform that enables developers to automate the deployment, scaling, and management of applications in lightweight, portable containers. Containers package an application and its dependencies into a single unit, ensuring consistency across different environments, whether it's development, testing, or production. Docker allows for faster development cycles, easier collaboration, and efficient resource use by isolating applications from the underlying system. It is widely used for microservices architectures and cloud-native applications, providing scalability and flexibility in software development and deployment.