Trapping is simply adjusting the lighter colors to spread into the darker colors to ensure there will not be any gaps or misprints due to human error or poorly registered paper. By overlapping the colors shared boundaries, it ensures proper printing in the event of misregistration. Seeing as most printing is done on white stock, if an object is not properly trapped the print could result in revealing the stock paper there the objects are not perfectly aligned.
Using chokes and spreads proper printing can be accomplished. A choke is applied if an item is adjusted to reduce the shared border. For example, if there is a 5mm knockout for a gray circle to fit in a larger black square, the knockout is reduced to 4mm adjusting the black square to choke the gray circle. A spread would be the opposite effect as it would be an adjustment where one of the objects is enlarged into the shared border of another. It would be making the gray circle larger to a 6mm diameter to accomplish the trap.
Resources:
Frank J, Romano. Pocket Guide to Digital Prepress. Clifton Park, NY: Delmar, Cengage Learning, 1996.
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