A set is referred to as printed materials (same stock and size) using one artwork or design (i.e. a front and back).
Think about a "set" as one person's business cards. If you are ordering cards for just yourself, you only need one set. If you are ordering for 5 people, you need 5 sets. Each person can have their own unique design for the front and back of their card.
The "quantity" you order will specify how many cards each person gets. If you are ordering a product with multiple sets, each person will get the same number of cards with the same order specifications (such as paper stock and coating).
As an example, if you order cards for 5 people and each person is getting 500 cards; you will need to order 5 sets of 500 quantity.