The "birthday reminder" column in your Clay workbook is likely a custom column you've created. Clay offers various column data types, including a Date type that accepts date and time ranges. This could be used for birthday reminders.
To understand the specific purpose and setup of your "birthday reminder" column, you might want to check its configuration in your workbook. If you need more detailed information about this particular column, you may want to request human assistance for a more personalized explanation.