What is the keto diet?   Is it a good way to lose weight?

The keto diet's central idea is to shift the body's energy source from carbohydrates to fat. 
 It centers around the restriction of carbohydrate intake between 30-50 grams per day.  The body's first choice is to burn carbohydrates for energy, but when carbohydrates are not present, it will revert to using fat as the energy source.  Fat from recent food intake, as well as from fat stores, is therefore used as the primary energy source.  This is a great way to lose excess stored fat.  But, you'll also need to have calorie deficit.