what is docker in docker code example

Example 1: what is docker

Docker is a set of platform as a service products that use OS-level
virtualization to deliver software in packages called containers. 
Containers are isolated from one another and bundle their own software,
libraries and configuration files; they can communicate with each other
through well-defined channels.

Example 2: what is docker used for

Docker is the best tool you can use if you want to develope one single
application that can run and work uniformly at any environment, laptop
or public cloud instance thanks to its convergent virtualized packages
called containers!!