The 'body fat setpoint' is a theory based on observations that: (1) people and other animals maintain a fairly constant weight despite not counting calories and day-to-day variations in calorie intake and expenditure; and (2) after periods of overfeeding or underfeeding people will return to roughly their previous weight (see next section).
Leptin is a homeostatic controller of stored body fat and the main driver behind the body fat setpoint. Leptin is released by our fat cells (adipocytes) roughly in proportion to stored body fat levels. Leptin can enter the brain, specifically the arcuate nucleus of the hypothalamus*, where it activates POMC neurons and inhibits AgRP/NPY neurons, both of these actions promote a decrease in appetite (food intake) and an increase in energy expenditure (see below)
* A part of the brain that is responsible for homeostasis of other things such as steroid hormones, body temperature and the circadian rhythm)
Ad Libitum means ‘at one’s pleasure’, or how much one chooses to eat when food is freely available
Defects in Leptin Signalling >> Obesity
ob/ob left, wild type right
But these mutations are extremely rare. Most human obesity isn't caused by these mutations and is characterised by very high leptin levels (due to having lots of body fat) and a lack of significant weight loss in response to leptin administration. This suggests that the defect in most human obesity in leptin resistance (LR) (analagous to insulin resistance) . When mice are fattened through diet induced obesity (DIO)* mechanisms of leptin resistance, such as various signals that inhibit the signal transduction of leptin , inflammation  and apoptosis  of hypothalamic neurons, occurs before obesity, suggesting leptin resistance causes obesity, but also maintains obesity.
So then, what causes leptin resistance?
* Obesity researchers need a way of inducing obesity in animals quickly and reliably. Many studies use a special diet called ‘high fat diet’ or ‘HFD’, which is high in fat but made from refined ingredients. They use it because mice/rats consistently eat many more calories on this diet . Note the difference between ‘high fat diet’ (a name) and ‘a high fat diet’ (a description of a diet that can be generalised). There’s enough evidence to suggest humans can lose weight on high fat diets and another way of explaining HFD is ‘diet induced obesity’ (DIO), so I’ll use DIO instead of HFD.