Intaglio means ink below the surface. Common intaglio plates are metal like copper, zinc or steel.  There are many processes for making marks, the most common using scribed opaque grounds to etch the surface of the plate.  After the surface of the plate has been etched, ground is removed and the plate is inked and wiped so the un-etched surfaces transferred to the paper remain white and the lines or patterns etched into the plate receive the ink.  Often the paper is wetted to grab more of the ink and emboss the line into the paper.