Jmod is the abbreviation of jeweler's monarch of deflecting, i.e. a particular combination of affixes on ashield which will benefit from the full bonus of the jeweler's affix (4 sockets), together with having a blocking bonus from the deflecting affix. Only one in about 15000 magical monarchs has that combination of affixes. They are usually used to insert (lightning) facets. You can't shop for them, as no vendor sells monarchs, so you either have to find them, cube them with the 3 pgem recipe or trade for one.