Question: I want a command to shred completely the contents of a folder/directory (which may be inside folders/directories).
Also please explain the command.

Answer: 
If you want to do this from Nautilus (aka 'Files' app), then you could use the [nautilus-wipe](https://launchpad.net/ubuntu/+source/nautilus-wipe) package.

* `sudo apt-get install nautilus-wipe`

Once installed, there will be two new options when you right-click on a folder: `Wipe` and `Wipe available disk space`. Choosing `Wipe` on the folder will give further options (e.g. number of passes, fast mode, last pass with zeros).
When I need to shred multiple files or an entire directory I simply use `shred -vzn 20 ./shredme/*.*` for example, which overwrites all files with any file extension in the "shredme" folder. Then you can use the standard rm -rf ./shredme command to remove the folder itself (or just right click and delete the folder) as all the data has been overwritten 20 times for this example.

I did a quick example of this with a bunch of duplicate images as an example. 

![Terminal screenshot](https://i.stack.imgur.com/s9ULy.png)