What is a null move and how to make it?

And what is the intention of playing it? Why does it exist?

1. e4 e5

2. cat Nf3 > /dev/null

Linux Bit Bucket Attack.

https://en.wikipedia.org/wiki/Null-move_heuristic

https://en.wikipedia.org/wiki/Null_move

Often a null move is a placeholder in a pgn file to attach comments to regarding a position. It is denoted by "--" in PGN notation and internally represents a move by the king to it's own square. As such it is an illegal move but as a vehicle for annotation it is extremely useful.

Some chess programs allow null moves and some do not. Usually it requires a special menu selection or key combination to insert one.

In a recent OTB game my opponent accidentally dropped her king. It fell on the floor and she picked it up, and put the king back. On another square! We both didn't notice it during the game. Luckily in the endgame her moved king simplified my win 

Back at home I discovered what happened when I tried to enter the game into Arena for analysis... The only way to enter the full game was to insert a null move.

I think that the main use of a null move is editorial.  For instance, in some endgame studies, you will see a specific move given for white (or black); however, the black (or white) move is given as "any."  This is where you would enter a null move.

As nicely pointed out by Willy Hendriks in his book Move first,think later, the null-move function can also be very useful for analyzing games,in particular for identifying the threats that one side is trying to execute.Hendriks provides the following position as an example:

Suppose the following position arose in one of your own games. You played something

like 2 2 . ..Tad8 but didn't manage to get an advantage and the game ended in a draw. Returning home late in the evening, you decide to put your engine on.

You notice that it likes 22 . . .Rg7.What's that good for, you mumble, and insert a null move: 2 3...Th8; aha, that was the idea, doesn't look too threatening though,one more null move: 24...Td5 ; mmm, he is really up to something, let's see, skip

one more turn: 2 5 ...Tdh5 .Ouch, the score is getting near minus 7 .