Stack of Cards

A Stack of Cards is a package of five pledge gifts, and is the first hierarchical combining of the pledge items given out by various Darnassus NPCs during the Love is in the Air seasonal event.

All of the Love is in the Air pledge gift packages are non-functional as regards their name and picture. You cannot read any of the cards in a Stack of Cards.

Source
If you have five or more  in your inventory, right clicking on any of them will consume five and create a Stack of Cards.

Ingredient
A Stack of Cards is one of three Darnassus pledge gift packages which are the ingredients of the Darnassus Gift Collection.