A refeed is a planned period of 1 to 2 days in the middle of a diet where you eat more calories, mostly from carbohydrates. Afterwards the deficit continues.
The idea is to refill glycogen stores, train better and get a short mental break. Unlike a cheat day, a refeed is planned and focuses on carbs rather than whatever you feel like eating. Unlike a diet break, it lasts days, not weeks.
What studies show
The evidence is thin. Campbell et al. 2020 compared, in 27 resistance-trained people over 7 weeks, a continuous diet with a deficit of about 25 percent against the same diet with two refeed days per week. Fat loss and weight loss did not differ. The authors report that fat-free mass and resting metabolic rate were better preserved with refeeds. A reanalysis by other researchers, however, found a difference only in dry fat-free mass. With so few participants, that is a hint, not proof.
A review by Peos et al. 2019 is similarly cautious: refeeds and diet breaks might blunt some adaptations to dieting, but evidence in athletes is largely missing.
What the scale shows afterwards
The morning after a refeed, the scale often reads about a kilogram higher. That is mainly glycogen and its water, since glycogen is stored with about three to four parts water. The jump has little to do with fat, and it fades once you are back in a deficit.
If your training suffers in a deficit, a refeed day before your hardest session of the week is worth a try. Judge it by your trend weight, not by the next day's reading.