"Internal rotation" and "external rotation" are terms used to describe the movement of body parts. "Internal rotation" is the movement in which a body part rotates inward. "External rotation" is the movement in which a body part rotates outward.
Example
Internal rotation of the hip occurs when the knee rotates inward. External rotation occurs when the knee rotates outward, for example when kicking a ball with the inside of the foot.
These terms are mainly used by (para-)medical professionals, such as doctors and physical therapists, to communicate more precisely about the mobility of a body part. As a patient, you do not need to be familiar with these terms, but they can help you better understand a medical report.