what is the meaning ? why it is used and how it is used ?
As far as I know, I can see that dev/full is a special device file unlike dev/sda or any other device files.
It is a unique device file which is always full.
Why is it used?
This can be used to test how a program handles disk-full errors.
How is it used?
You can simply try typing in this to the terminal to see how it works:
echo "Hello world" > /dev/full
Similarly, use it to see how your program responds when the disk is full.
Some links to research more on this: