The trapping process is a safeguard against possible (even likely) fluctuations in the printing process. Minor discrepancies in registration, slight paper shifting can cause inks to misregister.
When two abutting colored objects don’t meet exactly, you can end up with a printed result like the one below:
The overlap creates a dark area and the gaps cause an undesirable light area. To prevent these potential discrepancies, a trap can be implemented, causing the original colors to slightly overlap each other:
In general trapping is necessary when there are two abutting color areas that don’t share a common ink.
Placing a fine line where they join, made of a color that contains components of both abutting colors, can effectively trap such areas.