what the heck slots into back items???

insigs, sigils, gems, whatever - i can't seem to see what fits this socket??

Which socket? (A screen shot would be nice.)You need level appropriate Exquisite Jewels for anything Exotic or Lower quality, Ascended Back items don't take jewels, the stats are built right in.

If it says "Unused Infusion Slot", you fit an Agony Infusion or cosmetic infusion ( ie Snow Diamond Infusion). Normally items say what the unused slot is for.

Universal upgrades

Primary article: Universal upgradePrimary article: Gemstone

Universal upgrades, which include gemstones, are upgrade components that can be placed in any type of equipment. They provide smaller bonuses than components that are type-restricted, but they are easier to acquire than most other components.Jewels

Primary article: Jewel

Jewels are upgraded versions of gemstones created by a jeweler; they provide larger bonuses but can only be placed in trinkets.Runes

Infusions

Primary article: Infusion

Infusions are a special type of upgrade component used for non-amulet ascended equipment, legendary equipment, and some non-ascended back items. Enrichments are upgrade components that can be placed in ascended amulets.

https://wiki.guildwars2.com/wiki/Upgrade_component

Trinket mechanics

They can accept jewels as well as universal upgrades.Ascended versions can be infused.Back item gear slots are grouped with trinket slots in the Hero Panel.

https://wiki.guildwars2.com/wiki/Back_item

In case of below ascended back piece, you are looking for jewels/gemstones or universal upgrades.

In case of ascended, the stats are already added to the base item and a back piece offers 1 or 2 slots for infusions.

Insignias are used for crafting items.

Sigils are used for weapons.

cheers!!!!