Original: A resident physician or resident (also called a registrar in the United Kingdom and several Commonwealth countries) is a person who has received a medical degree (MD, DO, MBBS, MBChB) and who practices medicine under the supervision of fully licensed physicians, usually in a hospital or clinic.
Simplified: A resident physician or resident (also called a registrar in the United Kingdom) is a person who has gotten a medical degree and who practices medicine under supervision of other physicians, usually in a hospital.