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.
Damage: The data value for this item. The name "Damage" comes from when only tools used this value, now many other items use this value for other purposes. For blocks, it is the 4-bit "block data" tag that determines a variant of the block. Defaults to 0.
id: Item/Block ID. If not specified, Minecraft changes the item to stone (setting ID to 1 and Damage to 0, and ignoring any existing Damage value) 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.