Adaptive thermogenesis is the drop in energy expenditure that happens during weight loss over and above what the loss of body mass alone would predict. A smaller body naturally burns fewer calories, but during a sustained calorie deficit the body also turns down its resting burn a little further — an energy-conserving response to defend its reserves. The effect is usually modest, but it's real, and it's one reason weight loss slows the longer you stay in a deficit.
On a GLP-1, this matters because appetite suppression makes a deficit easy to sustain, so the "calories out" side quietly falls to meet the reduced "calories in" — contributing to a stall. Protecting muscle with adequate protein and resistance training helps defend resting metabolism against this drift. For the fuller picture, see calories in, calories out and what a calorie deficit is.