In Docker 101 - Introduction Overview for Beginners, we focus on Docker and Container. In the production environment, handling the container by hand is not a good solution.
Orchestrators are systems that reflect a declarative state in the environment and try to always have the right state.
There is multiple containers orchestrators like Docker Swarm, Kubernetes, Mesos/Marathon, ... but we focus on Kubernetes.
Definition Kubernetes.io :
Kubernetes is a portable, extensible open-source platform for managing containerized workloads and services, that facilitates both declarative configuration and automation. It has a large, rapidly growing ecosystem. Kubernetes services, support, and tools are widely available.
Google open-sourced the Kubernetes project in 2014. Kubernetes builds upon a decade and a half of experience that Google has with running production workloads at scale, combined with best-of-breed ideas and practices from the community.