A corporate cliche meaning that the direction of a business is ultimately determined by its customers. The business is compelled to sell products and services that customers want/need, at a price they are willing to pay, and provide an acceptable level of service, otherwise customers will look elsewhere and they will not make money.
It does not, as many people believe, mean that individual customers are always right, or that the business should be compelled to pander towards the whims of these individual customers. It is only the aggregate needs of all potential customers that need to be fulfilled.
If you don't think that sandwich is worth $6 then buy lunch elsewhere. The customer is king.