The sequence of screens and decisions a person passes through to complete one task: signing up, placing an order, submitting a claim. It is drawn as a map before any screen is designed in detail.
Most usability problems are structural, not visual: one step too many, a decision asked too early, a dead end with no way back. Flows expose those while they still cost nothing to fix.
We map the two or three flows that carry the product's value, count the steps and the decisions in each, and remove everything that does not change the outcome before moving to visual design.