That is a custom made Struct that gets created in an earlier video. You can see it here at 7:15 of this video:
To create a struct, right click in the content browser, highlight the “Blueprints” menu near the bottom and select Structure, then you can name your structure InventoryStruct, and fill it with the variables shown in the video.
In the video tutorial titled “UMG Inventory in UE4 – Part 5 – Scripting the Pickup Text and Inventory Menu” I’m working through, the tutorial guy starts talking about an array of “Inventory Structs”, but there is nothing coming up! Did this get removed/renamed before 4.6? What can I do instead?