A StackCP User is the account you create to give your customers access to manage their hosting and via StackCP - your customer's control panel.
Creating a StackCP User creates a username and password that you provide to your customer which will enable them to login to stackcp.com in order to manage their services. There is a guide on how to create a StackCP User here.
In the context of HostShop, they could be your paying customers who are receiving a service from you. When a new lead signs up from your shop, they’ll become your customer and a StackCP User will be created automatically and added to the list on the StackCP Users page.