Most permanent magic items must be attuned in order for their magic to become active, requiring a Meditation skill check with difficulty determined by the item in question. Attempting to attune an item takes about an hour, regardless of whether the process is successful or not. Temporary magic items, such as Evoked Objects, Talismans, Charms or Enchanted Items, do not require attunement to function.
Generally speaking each item attuned takes up a single Item Slot, and continues to occupy that slot until you attune something else in its place – although attempting to attune a new item and failing will still cause you to lose your attunement with the item previously using that slot. By default the number of Item Slots you have is equal to your Concentration.