Items can be found after successfully defending certain islands with a question mark attached to it. Each items have different uses to benefit you on your campaign. A campaign will always contain one of each items, including starting items. If you attempt to defend an island with an item on it and you fail then you will not receive the item, however you will be shown what it was. If an island is not defended you will not see what the item was.
There are currently 8 items: