Kingdom Hearts/Item Synthesis

Item Synthesis allows you to combine materials to make usable items and weapons. Unlock the ability to synthesize after you gain the Green Trinity ability in Agrabah. There should be a Green Trinity in Cid's Accessory Shop. After activating it, you'll be able to enter the Traverse Town Synthesis Shop through the ceiling of the Accessory Shop. You'll be able to synthesize from now on by talking to one of the moogles running the shop.

Synthesis Materials
Specific items have to be used to create items and weapons. These can usually be obtained in the forms of Heartless drops.