What is a controlled form field?

It's a field whose current value comes from the app's state. The component gets value, and the user's change calls a handler that updates the state. That way I can check the data, show an error and control the field's value.

An example is a search box with value={query} and query updated in onChange. An uncontrolled field keeps its current value mainly in the DOM; the app can read it when needed. Form libraries often combine the two approaches to avoid unnecessary work. Make sure you don't accidentally switch a field between controlled and uncontrolled, e.g. by changing undefined to a string.