Erpnext attendance

/api/method/erpnext.hr.doctype.employee_checkin.employee_checkin.add_log_based_on_employee_field

what is this for ?

I have an existing biometric attendance system, and I want in/out from it, how can i connect them.

Please check the docs:

https://docs.erpnext.com/docs/v13/user/manual/en/setting-up/articles/integrating-erpnext-with-biometric-attendance-devices