ECFMG stands for Educational Comission for Foreign Medical Graduates. A certification by ECFMG is a standard of evaluating the readiness of an International Medical Graduate to enter US Graduate Medical Education and provide patient care. In short if you want to study further or join a residence program or practise in the US, a certification from ECFMG is mandatory. You have to apply for an Identification number from ECFMG for appearing for USMLE. You can check the complete procedure at: https://iwa2.ecfmg.org/appforcert/onlinedocs/afcoverview.aspx