What exactly is a pocket item? A pocket item is one category of items that can be unlocked by the Tome of Knowledge. Usually it entails killing a special named mob of some kind. Since this notification can be easily missed, a good way to check for any items you are qualified to get is to open up your ToK and click on the Rewards tab on the left side.
Then click on the items submenu to see all the items you have unlocked so far in the game. If you have not yet done so, now is the time for you to make a trip to the Lyceum in the Inevitable City or the Library in Altdorf to purchase the item itself from the appropriate vendor.
After you have bought the item, you will then need to equip it in a pocket item slot. You can only equip two pocket items at a time. I guess Mythic didn't want us to have too many of these overpowered items active at any particular time. You will also need to drag the item to a hotbar slot in order to activate it. Activating it is as simple as left clicking on the hotbar slot button itself.
All pocket items have an emote associated with it that informs you and everyone around you what happens when you activate it. Some of them even have special animations and sounds. Pointless fun and great for any collectors out there.