It refers to the girl or the lady who has a birthday, the agazajada. Of the birthday.
In men it is birthday. It is who is years old the day of the birth of each one. Normally it is celebrated that day with great joy and enthusiasm and that person receives gifts and gifts of people who want you or appreciate.
A birthday girl is a woman that is in your day's birthday. When is serving 15 years are you called quinceañera.