From Minecraft Wiki
Count: Number of items stacked in this inventory slot. Any item can be stacked, including tools, armor, and vehicles. Range is -128 to 127. Values of 1 are not displayed in-game. Values below 1 are displayed in red.
Slot: The inventory slot the item is in.
id: Item/Block ID. If not specified, Minecraft changes the item to stone when loading the chunk or summoning the item.
tag: Additional information about the item, discussed more in the subsections of the item structure section. This tag is optional for most items.