An ERAS token is a code that ECFMG issues to IMGs so they can register for MyERAS, the application system run by AAMC (ERAS). It connects your ECFMG record to your application so your exam results and certification status can reach programs.
You request it in the MyIntealth portal under ERAS Support Services, and you need a new one each application season. One token covers all the specialties and programs you apply to in that season. Tokens from earlier seasons are not accepted, and a fee applies (see ECFMG's page for the current amount).
US medical graduates do not use ECFMG tokens. Read the full guide: The ERAS token: how IMGs get it and when. You will also need a verified MyIntealth account first.