You will need the Full Write Forms permission to perform this action.
Conditional fields are able to show and hide fields based on responses to other fields in a form.
If the option named "Is Conditional?" is set to Yes, you will be able to choose another field on the form and which responses would cause that field to appear. Until that specific response is made, the field will remain hidden.
Fields you select for this are limited to dropdowns, radio button lists, checkbox lists, and yes/no fields.
You could have a field named "How did you hear about us?" with various options including an "Other". Only if the user selects "Other" will a textbox appear where they can enter more details.
Conditional fields with the Grouping Field
This field will allow you to group several related fields together. This is useful if you want to conditionally make all of those fields appear together. You would set the condition on the grouping field itself in this case and either all the fields will appear or none. This saves you from having to set each field individually.