IMG stands for international medical graduate. In the US residency system it means a physician who graduated from a medical school outside the United States and Canada, whatever their citizenship. A US citizen who graduates from a school in the Caribbean or Europe is an IMG, and a non-US citizen who graduates from a US school is not.
IMGs must be certified by ECFMG before starting residency, and they apply through ERAS with an ERAS token. Programs differ in how open they are to IMGs, and many look for US clinical experience.
ECFMG has stated that graduates of Canadian schools on or after July 1, 2025 are considered IMGs for entry into US GME, so check current rules if that applies to you. See the ECFMG certification overview.