The contents of each bundle will vary from month to month, and will include items such as Poké Balls and Lucky Eggs, amongst others. The announcement specifically describes the bundle for July being "extra-special", though goes into no specific details of what will be included.
Trainers who join Amazon Prime, including those who join with a free trial membership, will receive immediate access to whatever bundle is currently available, as well as all future bundles during the period of their membership. Each code will expire one month after release. Once the code has been claimed in-game, all items and bonuses are kept by the player regardless of if their Prime membership and/or the itself code expires before they are used.