What is dev/full?

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.

To know what device files are - learn about the Linux File Hierarchy here, you can also check out the details of dev/full in the documentation.

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: