Ineffable insignia and inscriptions are used to craft leveling equipment with selectable stats.
They are pretty pointless since they are so expensive to craft, leveling is so fast, and content is so easy at low levels that whatever you craft with them is a soon outdated waste of resources.
It's one of these weird things players wonder why were they added at all since it's pretty much pointless.
I have this in my "daily" bookmark folder. So you've got at least the second it takes for me to check what's new in here.