Items can be found during Main and Side Encounters, Boss battles and Adventures, while you can also Craft them. Each Hero can use up to four Items. Each Item enhances the Hero's parameters in some way.
There are 10 Item Levels in total, and you can equip your Hero only with Items of the Hero's current Item Level.
All items are stored in your Inventory. You can find the Inventory in your Hero window in the bottom part of the window.
How to equip items
If you have items your hero can equip then in the Heroes window (where your collection is) there will be the green plus (+) sign on the portrait of the hero for which the item is available. To equip the item press the hero portrait twice and then touch the slot with the same green plus (+) sign.