Adobe ACROBAT 3D Manual page 606

About trapping
When a commercially printed document uses more than one ink on the same page, each
ink must be printed in register (perfectly aligned) with any other inks that it abuts, so that
there is no gap where the different inks meet. However, it's impossible to ensure exact
registration for every object on every sheet of paper running through a printing press, so
misregistration of inks can occur. Misregistration causes an unintended gap between inks.
You can compensate for misregistration by slightly expanding one object so that it
overlaps an object of a different color--in a process known as trapping. By default, placing
one ink over another knocks out, or removes, any inks underneath to prevent unwanted
color mixing; but trapping requires that inks overprint, or print on top of each other, so
that at least a partial overlap is achieved.
Misregistration with no trap (left) and with trap (right)
Most traps employ spreading--expanding a light object into a dark object. Because the
darker of two adjacent colors defines the visible edge of the object or text, expanding the
lighter color slightly into the darker color maintains the visual edge.