JDM stands for Japanese domestic market.
Originally it was a phrase coined for the Japanese market hot hatches from ther likes of Honda, Nissan etc. These models were available in Japan only and featured subtle differences to the western versions, and were more concerned with performance.
These parts started to become available in the western markets for people who wanted the look, but was still for Japanese brand cars.
It now seems to refer to anything which comes from Japan, and seems a bit funny to be available for Europena brand cars.
Something like that anyway, sure someone will correct me if i am wrong.