The carbs in your diet supply you with a fuel source - glucose. Once your short-term glucose needs are met, your body converts leftover glucose into a more compact substance called glycogen, and then stores this in your liver & skeletal muscle tissue. When you need more fuel to keep going, your glycogen supply is reconverted to glucose. You can only store so much glycogen in your muscles and liver. If you still have excess glucose in your system after your glycogen reserves reach maximum, your body will convert the remainder to triglyceride and store it in your fatty tissues.
If you don't consume enough carbohydrates to fuel your body's need for glucose, your body will use stored glycogen and fats to make glucose. This process forms a by-product called ketones, which puts your body into a state of ketosis.
Mild ketosis is a proven method for sustainable weightloss, extreme ketosis can cause problems which is why you see it plastered everywhere.
One last thing to consider.....After carbohydrates and fats become depleted, your body will break down muscle, so while on a ketosis diet its important to do strength training to maintain lean muscle mass.