It's like a virtual machine, but doesn't require as much resources. Everything that's needed to run an email server is packaged inside the container so you can just start it up and things work out of the box.

It's a way to isolate an app and it's dependencies. This is done for security reasons and portability, but I suspect it has more to do with the fact that most developers are too lazy to make something work bare metal on every platform there is out there.