Trapping digital files is the process of compensating for misregistration on the printing press by printing small areas of overlapping color where colors meet.
If you are printing on an ink jet printer, the color goes where it should. When printing on a press, registration problems can occur easily with each run through the press. That means if you have colors butting together, they may not register properly and a thin white gap will appear where the colors were meant to meet. Trapping amounts can be set in the layout program to overlap color and avoid this problem.