5:2 is a way of intermittent fasting. If you don`t know what fasting is, check out my Beginners guide for intermittent fasting here.
I think this is the easiest way of fasting since you only fast 2 days per week and you eat normally for the rest of 5 days. You can choose which days you fast and which you don`t. You can fast Monday and Friday and eat normally for the rest of the week.
Keep in mind that normally doens`t mean fast food, it means you can eat 1500 or 2000 calories per day. Try to eat healthy those days too.
The days you fast , you can eat 2 meals of 250 calories ( for women ) and 300 calories( for men), or you can split your calories however you want between your meals.
Try to avoid sugary drinks and only drink water, coffe and tea.