A user flow is a picture or outline of the steps someone takes through a product to finish a task. It is like a map through a building that shows turns and possible choices.
A sign-up flow might go from the welcome page to an account form, then to a confirmation screen. If someone cannot complete a step, the flow can help a team see where the path needs attention.
A flow focuses on the steps inside the product. It may not describe everything a person experiences before or after using it.