Item tables are a reference to pre-set units I guess, which you clicked in the World Editor, got to Items Dropped tab (?). Item tables define an order of items, that will be dropped with a specified chance, once that specific unit dies. Let's say you have 10 unit-types placed on the map, but you want those units to drop the same item-types. If the item-types dropped are too many, then, the Item Table will save your time (and your patience). So, instead of clicking every single item that a unit will drop each time, you can refer to the items dropped instantly throughout an item table.
E.g.
Your golem drops with 100% chance a Chest Key. It will also drop for example with 20% chance a Periapt of Vitality, a Ring of Regeneration with 19% chance, a Stone Token with 10% chance and a Ring of Protection (+4) with 57% chance. Rename the table to Table1. This "Table1" currently contains five drops: Chest Key with 100% chance, a Periapt of Vitality with a 20% chance, a Ring of Regeneration with 19% chance, a Stone Token with 10% chance, a Ring of Protection (+4) with 57% chance.
If you create another unit (let's say a murloc) and you want the same drops, you will go to "Use Item table from Map" and select the Table1, so that the new unit automatically drops the same stuff, instead of re-typing the chances and selecting the items over and over again.