A private label product is manufactured by a contract or third-party manufacturer that is sold under a retailer’s brand name. As the retailer, you will specify everything about the product—how it’s packaged, what the label looks like, anything else that goes with it—you will pay the manufacturer to have it produced and delivered.
The opposite of this is buying other companies products (with their brand names on them) and reselling it, which is known as arbitrage.
Private label is mostly synonymous to a white label product. Where a white label product is a product or service produced by one company (known as the manufacturer or producer) and other companies (known as the marketers or entrepreneurs) rebrand the product as if they had made it. The only real difference is that with private labeling, you have more choice.