Starvation mode happens when you aren't getting enough calories. It means the body becomes super efficient at making the most of the calories it does get from food and drink.
The main way it does this is to protect its fat stores and instead use lean tissue or muscle to provide it with some of the calories it needs to keep functioning. This directly leads to a loss of muscle, which in turn lowers metabolic rate so that the body needs fewer calories to keep ticking over and weight loss slows down.
Of course, this is the perfect solution if you're in a famine situation. But if you're trying to lose weight, it's going to do little to help you shift those unwanted pounds. It actually makes you lose weight much slower.
Hope that helps.