These are thin vertical or horizontal lines which appear in the margins or inside the design of the stamp.  The vertical guide lines may often be seen in or near the star corners or the check letter boxes.  The horizontal guide lines are often visible in "Postage" or the value tablet as well as in the corners. The guide lines are placed upon the plate previous to the laying down of impressions to allow for proper alignment during application of the transfer roller.   Guide lines appear on the penny Blacks as follows:

Vertical Guide Lines

These appear on a great number of all 11 plates of the Penny Black stamps.  and are most frequently seen in the top right north-east corner squares.  They appear with great frequency on Plate 8 and 9, especially on Plate 9 where they may also be seen in the south-east corner as well as the north-east.  They may also be seen along side the frame lines or close to the outside of the corner squares.
 
 

Hypothetical example of vertical guide lines indicated in red.
 

Horizontal Guide Lines

These can be seen running through all of the corner squares as well as the value tablet and "Postage."  They may be found along the upper or lower margins, particularly on Plates 2 in the S  row and Plate 9 in the K row.