By default, when you print opaque, overlapping colors, the top color knocks out the area underneath. Overprinting prevents knockouts and makes the topmost overlapping printing ink appear transparent in relation to the underlying ink.
Where colors printed from separate plates overlap or adjoin one another, press misregistration can cause gaps between colors on the final output. To compensate for potential gaps between colors in artwork, print shops use a technique called trapping to create a small area of overlap (called a trap) between two adjoining colors. You can use a separate, dedicated trapping program to create traps automatically, or you can use Illustrator to create traps manually.