A crash diet is a diet which is extreme in its deprivations - typically severely restricting calorie intake. Meant to achieve rapid weight loss, a crash diet differs from outright starvation only slightly. Crash diets are typically unhealthy and are rarely - if ever - recommended by doctors or dietitians. Crash diets can lead to malnutrition, and are not a recommended means of weight loss.
After a person discontinues a crash diet, the "yo-yo effect" is often seen. This causes a person to eat far more than normal, causing them to regain both the weight that was lost due to the diet as well as additional weight. This is caused by an evolutionary trait of the human race that historically only took effect in times of famine: after a famine ended, people's bodies naturally craved to regain both the weight that was lost, plus additional weight as well, in order to protect themselves in case of another famine.
Starving your body on a crash diet is not only very unhealthy, but in most cases people who have a rapid weight loss by crash dieting or other unhealthy means usually gain it all back. This is because crash dieting does not help you to change the habits that have led to your weight gain. Instead, take a more gradual approach to weight loss and develop a weight management strategy that you can maintain for a lifetime. This will help you lose the weight, and give you the power to keep it off.