A loss leader is a product that drives customers by the millions into you store (physical/online etc.) and involves selling a product or service at a price that is not profitable but the only objective here is to attract new customers. These new customers will eventually also buy something to go with this product or other products which will more than compensate for the loss in this product category.
E.g. Razor and Blades : Sell Razor at below cost or cost price so that customer gets attracted with the price and buys the Razor, but the razor is useless without the blade (this is where the company will make money).
Another example would be gaming consoles, where the consoles are sold at below profit or at cost so that with every new game being launched the console owners would eventually have to buy the game from the console manufacturer (brand) itself resulting in incremental and recurring revenue for the brand.
In Platform businesses, this loss leader concept is very much evident e.g. payments business, where the platforms only objective is to attract customers and merchants to the payment platform where the upsell (lending) will make most of the money for the platform.