How does summon item work in 1.12?

In order to summon an item, you have to add a data tag to specify which item to summon and the quantity of that item.

Here is the command:

/summon Item ~ ~ ~ {Item:{id:"<item>",Count:1b}}

Replace <item> with the item to summon. Must be a valid item id, or block id for which items exist.

Here is an example command which summons a diamond sword:

/summon Item ~ ~ ~ {Item:{id:"minecraft:diamond_sword",Count:1b}}

You could also use a minecraft item generator which supports summoning items.