Basically this is what makes people buy your product, it could be uniqueness, quality, quantity, service or price. Usually this is pretty dependent on what you are selling, but it is important you get it right. in the case of our mandarins it was fresh fruit delivered at good price. I'd say there were a few other factors influencing the value proposition, but we'll get to that in a moment.
In order to find your customer and then make a sale, you need to understand what it is they want then deliver it again and again. There are plenty of people that offer similar services all over the world, but the successful ones understand what the customer wants and make it very easy for them to get it. No matter what you decide to sell, your value proposition needs to hit the mark in order for you to succeed.