Basic concept

If you add a text input component to the canvas, you will be asked to specify its label, for example First Name. If you then select this component and go to the General section in the attribute bar, you'll notice that the Value attribute is already defined as a form expression with the value set to {{firstName}}.

The text value entered in the text input field is stored under this {{firstName}} variable, which you can use in other components to build dependencies between form fields.