Its a spell/recipe you learn (from the trainer or a drop, dont remember) around 375? i think tailoring. The way it works is that the tailor can loot extra frostweave from random mobs, in the regular outdoors its pretty much 1 extra frostweave every 4th-7th mob, sometimes more, sometimes less. In instances mobs can drop x4 - x7 (ie 7 extra frostweave) for the tailor alone, again its not suuuper frequent but when it does happen you do a little happy dance.

The thing that really sucks is that only the tailor can see/loot it, since most of us FFA the loot and only loot on our controller you will easily forget to loot most of it. I occasionally swap to my mage (tailor) window and just look quickly for any sparkles, but usually only after I've killed a bunch of humanoids in a single spot and can check them quickly.