Obesity is a medical condition when a person or animal weighs too much and the excess weight is due to fat. Obesity increases the risk of heart disease, cancer, arthritis and other illnesses. Obesity significantly shortens life expectancy. Some people who obese are so because they have a medical condition that causes it, however most just eat to much and/or don't exercise enough.
Irresponsible manufacturers sell food that is high in calories, that tastes good but causes obesity and other health problems. If the law allows this they'll try and prevent their customers knowing how fattening their products are. The Fast food industry is notorious for this.