A form of shading in ink or pencil drawing\sketches, where one creates shadow or the suggestion of colour by drawing lines diagonally in one direction, and then cross-hatching them with similar diagonal lines passing in the other direction.
The more lines you do the darker the shading becomes.
Of course, I just clicked this link off the home page. It could have a whole other slang meaning I don't know.
Neatly spaced usually diagonal lines to indicate different shades or shadows in a black and white pen and ink drawing.
Use crosshatching to show the shadows.
The # character also called:
Press the key with the crosshatch on it.