Controlling fields can be select lists, checkboxes and Salesforce Lookups.
Any form element can be made dependent on one of the above. So you can not only control text or number field visibility, but rich media and signature elements as well.
To add a dependancy for a particular form element, you will need to click the pencil icon on the form builder element.
Click on the "Add Controlling Field".
On the next screen you will be able to select the controlling field and the value of the controlling field that will enable this dependant field. Select the value and click "Add".
Save you form when finished editing. Now on the form you will be able to control the dependant field visibility.
Dependent field not displayed.
Selecting the right value will display the dependant field.