A microflow is a custom piece of logic, a “function” if you will. This can be anything from a validation flow, to calculation, to API calls, sending e-mails, etc. Once a microflow is kicked off (by a user, or by a background process like a scheduled event) it keeps running until it's done.
https://docs.mendix.com/refguide/microflows
A workflow is a way to model a business process. They are steps that are automatically paused when user input is required. They can also include microflows. You can read more about workflows here: