Taken as inspiration from APIGEE:
"From a business perspective, you can think of API products as your product line. You create an API product to bundle your APIs at particular service levels for client app developers. Different API products – perhaps even bundling the same APIs – provide service levels for different use cases. Instead of just giving developers a list of resources, you can bundle specific resources together to create a product that solves a specific user need.

Client apps are how developers access your API products. When a developer registers an app, they select the API products to associate with the app, and the solution generates an API key for the app. By default, a single key provides access to all API products associated with the app.