A set point is a theory that states everyone's body has a genetically determined range of weight and temperature that their body will try to maintain to stay at optimal health. This means if Susan has a weight set point of 136 pounds, her body will try to stay around that weight. If she eats less and exercises more, her body's metabolism will slow down. If Susan eats more and exercises less, her body's metabolism will speed up. This does not mean Susan can't ever weigh over 145 pounds or under 120 pounds. It means that her body will attempt to compensate for Susan's eating habits in order to stay in that range.
Proponents of this theory encourage individuals to stay at their body's set point and to accept it despite social pressures to be thin. They believe that the reason people have difficulty losing or gaining weight is because their weight goals are outside of their set point range.