Form fields are data fields you use in your Action Forms pages to gather information. They can be a variety of types (text, dropdown, etc.) with multiple customizable properties (conditions, validation, etc) and can be used in further actions.
You can also reorder them with drag&drop in the Layout Mode. To remove form fields, you have to select one or more fields, then click the Remove button at the right and then click on the Save button. You can expand the fields just by clicking on the label, this will open the settings panel where you can customize the UI.
The main configuration options are the Title and Type fields. Basically, the Title is the label of the the form. The Type field is a drop down list which specifies the input type required from the user, for example Text, Email, Check box, etc.
You can customize the form fields with the General, UI Settings and Validations options. The General options allow you to select the type of the field, name, assign values to the title, etc. The UI Settings option allows you to apply CSS styles and the Validation option allow you to set the filed as required and to put specific validations on it. See the screenshot below (this is an example of Text Box type of field).
For additional information about fields, please check each field page. Please remember that Add-ons will add their own Field Type with their own actions, and those below are the default ones of a clean installation of Action Form.