Just as container ships can carry more cargo because they use a standard form factor for storing the cargo, software containers can run more apps on each server by using a standard way to share the operating system without all the wasted overhead of virtual machines.

How?

1. Containers bundle together all the pieces you need to run the app into one package.
2. Containers make it easy to deploy apps to local servers and the cloud.
3. Docker makes it easy to create containers.

Also check out Steven J. Vaughan-Nichols’ What is Docker and why is it so darn popular?.