Postmates is an on-demand delivery service founded in 2011. Postmates considers themselves a logistics company getting food, groceries, and alcohol from point A to B. The way Postmates makes their money is by charging a delivery fee for food which can be paid by the consumer/customer or the restaurant (most likely to get more orders).
As a Postmates Driver, your main job is to pick up food from a specific location and deliver it to a specific customer. The majority of these locations are restaurants, bars and grocery stores. You are getting paid essentially to be a delivery person that deliveries food in great shape. The better at delivering a product in a great shape, the more tips you make and the more money that is in your pocket.