Placeholder Text allows you to add predefined existing text that will be overwritten when the registrant starts filling in the text field. This is most common as an added instruction for registrants to understand what they need to enter.
An example of utilizing placeholder text is asking registrants to enter their first name.
Once a registrant starts typing in the field, the "Enter Your First Name" will automatically disappear. If the registrant does not do anything to the field, the field will be submitted with the default placeholder text you've written.