A Registrant is the person registering for the event. An Attendee is the person attending the event. For example: You are registering for yourself and your partner. You are the Registrant. You and your partner are the Attendees.
A Registrant could also be an Attendee or could not be attending at all. For example: an executive assistant could be registering for their boss. In this case, the assistant is the Registrant and the boss is the Attendee.
By default, EventCreate collects information about all Attendees.
If you wish to modify this Data Collection option, go to Setup > Registration > Advanced and choose to collect information from Attendees, Registrant and Attendees, or Registrant Only.