The supply chain is a huge system of organizations and individuals that is involved in creating a good or a service that a customer wants. The supply chain involves large numbers of people and can stretch across the world.
A supply chain can be said to start with an individual consumer who wants some good. They go to the store to buy that good, making the store and all its employees part of the chain. The store got the good from a wholesaler who is also part of the chain. So is the transport company that got the good to the wholesaler and the company that made the good in the first place. From there, we can go even further back to all of the companies that provided any parts that were needed for the good and the companies that provided the raw materials to be made into those parts.
In other words, any individual or organization that has a part in creating the good and getting it to the consumer is part of the supply chain.