Q: What is trapping?
A: The ability to print wet ink film over previously printed ink. Wet trapping is dependent upon several press and paper conditions, including hardness and holdout of the paper, tack of the inks, and general condition of the rollers, cylinders and blankets on the press. Dry trapping is printing wet ink over dry ink. Improper trapping will cause color changes.

OK now in English - Trapping is basically bleeding one color into another color when two colors touch.
How it looks printed
Close-up view of the trapped area.
In the image above, you can see a slight color variation above the arrow-this is the trapped area. Now, you might think that things are going to look funny because of the two colors blending like that, but it won't. The trap size is very small and it prevents leaking, which is noticable.
(See: What are the benefits of trapping a job?)
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