the short answer
A refeed day is a planned day within a calorie deficit where intake is raised - mostly through extra carbohydrates - to bring the day up to roughly maintenance calories. Protein stays at its normal target and fat stays low. The goal is to refill muscle glycogen and give the diet a short, controlled break.
This article is general nutrition information for coaches, not medical advice. Coaches coach nutrition habits and targets - if a client has a medical condition, a clinical nutrition need, or a history of disordered eating, refer them to a registered dietitian or doctor.