Imbuements are temporary properties that can be applied on certain pieces of equipment. All imbuements last for 20 hours and this time only counts when the equipment is equipped and in use (in battle for most of them, but all the time for capacity and speed imbuements).
There are 3 levels of imbuements: Basic, Intricate, and Powerful. To imbue an item, you first must start the Forgotten Knowledge Quest in the Temple of Light north of Thais. You will need Astral Sources (creature products) which will be used to imbue the item. Basic imbuements require certain amount of one item, intricate of two items and powerful of three different items.
To imbue an item, go to any City temple and use the Imbuing Shrine on it. Select one imbuing slot that is available, the desired imbuement and its level. Depending on the price of Astral Sources it may be worth it to pay the additional gold fee to have 100% of success rate (otherwise you may fail and lose the Astral Sources).