To understand Customer Journey Design, you first need to understand the term "customer journey."
The Customer Journey is the collection of experiences, desires and feelings your customer has from beginning to end. It may also include the customer's experiences for sometime before they even hear about your company for the first time, as well as for some time after the customer no longer interacts with your business.
Customer Journey Design looks at the experiences, thoughts and feelings of past and present customers, and analyzes how those experiences impact your efforts to serve the customer. The design also makes recommendations on how you and the customer can work together to the benefit all concerned.