Message flow in a diagram refers to the exchange of messages or information between different components or entities within a system. A message can be any type of information, such as data, requests, commands, or responses.
For example, in a system where a customer places an order for a product on a website, the message flow might be illustrated through a line connecting the customer and the website to represent the submission of the order. The message flow might then continue with a line connecting the website and the warehouse to represent the order fulfillment process.