Items are a type of Gear card that can only be played once within scenarios. Items are equipped in the Item slot, of which each PC has one. Once an Item is used, it should be flipped over to signify that it is no longer available for the remainder of the scenario. A PC can use their Strategy Phase to swap a used Item for an unused, unequipped Item.
Used Items should be flipped back over once the scenario ends. They are not permanently lost.
Items come in three varieties: Combat Items, Exploration Items, and Interaction Items. Items can only be used in their corresponding scenario type. The equipped Item can be changed at the start of a scenario to ensure that it is of the correct type.
No training is required to equip Items.
Items are often expendable in nature. However, in Triptycho they are recovered at the end of each scenario. The typical explanation for this is that the Wealth cost for the Item includes the ability to continue to resupply the Item, including having some extras available that the PC can dig around for between scenarios to make an Item available for use again (or having the raw materials needed to construct another).
In other cases, particularly with Treasure, Items inherently recharge (typically with magic). Recharge time is not specified, but time between scenarios is generally enough to permit this.
The DM may exert more control over this resupplying if these explanations are found unsatisfactory and/or unrealistic for a given campaign. The DM could rule that the PC must be able to create or purchase a new copy of the Item before allowing it to be used again (though this should not cost additional funds as a single expendable Item is certainly not worth its listed Wealth cost). The DM may also provide a specific time requirement for Items that recharge, especially for Treasure provided. Doing so adds additional bookkeeping to the game, however.
For more information, see Player's Guide Chapter 7: Gear and Wealth.