Intermittent fasting is when someone is going longer periods of time than normal without eating food.
Not long ago this was something humans did on a regular basis, if you had an unsuccessful day hunting/fishing/gathering food then you didn't eat! This allows your body to burn up all the readily available sugar/carbohydrates in your system and then switch over to burning fat and protein for energy. Most people have more than enough fat on their body to not worry about burning the protein/muscle on their body for energy.
Every night when you go to bed your body is going to be switching over to ketosis for a short period of time before you eat again in the morning. After you eat breakfast your body is now using carbohydrates for energy again given you ate something that has carbohydrates.
When your body switches over to ketosis during a fast it will first start to burn the extra fat stores on your body. The fat you store on your body actually has a purpose! It's there for times when there is no food around, but in this day and age there will always be food around so we just keep on storing it as fat.
Research has shown intermittent fasting to be very valuable from a weight loss standpoint and even potentially in different types of cancer treatment when done properly. Extra sugar is stored as fat on the body, and many forms of cancer use sugar as their main source of food, so it just makes sense!
You don't want to stay in a fasted state long term though because your metabolism will drop and cause other long term problems. just doing it for a day here and there is the best way. Plus if you do it long term you won't have the energy to work, workout or be active.
The best way to do any sort of fasting would be to do it on days where you aren't exercising or as active as normal. Try eating less food on your inactive days and don't eat right when you wake up in the morning or right before bed. Keep your meals to the middle of the day so the length of time between meals is the longest. The day before and after a fast you would want to go back to eating like you normally would.