Scribers are hand tools used in the metal trades to mark lines on workpieces prior to machining.

Due to the nature of metal, and the accuracy generally sought after when machining it, a thick pencil or ink line would either be impossible to accurately lay down or prone to erasure during the machining operation. A crisp bright mark exposed by the scratch of a scriber is semi permanent and thin. The process of using the scriber is referred to as marking out and may require the addition of other hand tools and equipment.

At the very least a layout stain of some form is generally used to contrast the scibed line against the background surface.

A scriber block is used to layout lines at a set height from the base, thus it's second name surface height gauge.
The work piece is held against an angle plate so that it is perpendicular to the surface plate, the scriber block is then adjusted to the required height and used to scribe a line parallel with the table, by sliding the block along the tables surface.
The scriber block has pins that can be pushed to protrude through the base, these can then be used as limit pins to allow the block to press against the edge of the table, controlling it's movement in that plane. The upright post can be adjusted to tilt back or forward, effectively moving the scriber point up or down in a controlled fashion, coarse adjustments are made by sliding the scriber physically along it's clamping block.
The scriber block may also be used to detect run out (a variation in concentricity) of a workpiece mounted in a 4 jaw chuck. The scriber point acts as a reference against which any variation in the work piece can be compared against, and adjusted for.