What does one-way data flow mean in React?

Direct answer

One-way data flow means data is normally passed down the component tree through props, while children request changes by invoking callbacks or updating shared state through an owning abstraction.

Parent owns data: the parent decides the current value.
Data flows down: children receive current values through props or context.
Events communicate intent: children can call handlers supplied by the owner to request an update.

This model makes it easier to identify the single source of truth for each piece of application state.