Socketable Items are some very special and rare items that can be found into level 40+ Dungeon & Raid Boss Corpses or into level 40+ Dungeon & Raid Boss Chests (See Nordock Dungeons & Raids for more info).
These items are unique, because it is for you to decide what properties to put on them, indeed, these items are looted completely empty.

To put a property into one of these items, you also have to drop some specific Socketable Gems:

• These gems can also be found into level 40+ Dungeon & Raid Boss Corpses or into Dungeon & Raid Boss Chests.
• Each of them occupies a slot into the Socktable Item (gems of the same type do stacks, and occupy the same slot, see below)
• Gear items have a maximum of 6 slots on them, weapons slots scale based on weapon type (Check here for scaling details)

Weapons Limits:

• Max Damage Bonus & Massive Critical: 2d6 from each source
• Max Attack Bonus: +6
• Max Enhancement Bonus: +6
• Max Vampiric Regeneration: +6
• Max Mighty Bonus: +10

Gear Items limits:

• Max AC Bonus: +6
• Max Damage Resistance from each source: 15/-
• Max Regeneration on each item: +4
• Max Ability Bonus of same type on each item: +6
• Max Saving Throw Bonus of same type on each item: +5
• Max Skill Bonus of same type on each item: +15
• Max Spell Resistance: 32

If you want to get rid of an unwanted socketable item, you can scrap it inside the Special Forge in Epic Zone - Castle Gledario.
If you want to remove a property from a socketed item, you can do that by using the "de-socket anvil" in Epic Zone - Castle Gledario.
See Special Forges for more info.

The following list will give you an idea of the properties you can socket on these items (properties that stacks will occupy only 1 slot on each item):