The clue is in the title. It is a diet plan where your normal meals are replaced.
This replacement can be a pre-packaged meal or a liquid meal, in the form of a shake, or a snack-type bar. You will usually replace one or two meals a day with these. Because these replacements are high in protein and low in carbohydrates and calories they will leave you feeling satisfied. In fact if you follow the manufacturer's advice and additionally drink large quantities of water you will feel quite full and not need those in between meal snacks.
Since two replacement meals per day will probably only supply 6 - 800 calories almost all will suggest a healthy nutritious meal per day in addition.
This normal meal will not only provide some additional nutrients and minerals but the extra calories will in fact help you to lose weight. As strange as that may sound, if you put your calorie intake at too low a level you run the risk of your body going into shut down mode. This is where the body sees the reduced energy (calorie) input as impending famine and hangs on to its stored reserves. These reserves are of course your body fat, the very thing that we are trying to lose.
When looking at the companies providing replacement meals you will find their web sites loaded with testimonials from satisfied customers. I am sure that these testimonials are, for the most part, real, but you should instead concentrate on the ingredients of the product. Do see that you are getting the nutrients that you need for a balanced diet. As I have said frequently elsewhere if you can't pronounce it or don't know what it is, don't put it in your mouth.
The promoters of replacement meals will have put together a product that they hope will contain the vital protein, carbohydrate, minerals and nutrients that you need to assist you in your pursuit of weight loss. There are however a few things that you should bear in mind. What your Granny said about eating less and moving more as the answer to losing weight is still true. Don't start any revised eating plan that does not include additional exercise.
Furthermore, if you use replacement meals or any other weight loss plan to achieve your ideal weight then you are to be congratulated. But all the effort will have been wasted if you then resort to the life style that got you overweight in the first place.
Take the time to learn about real food and sensible eating. Keep up the exercise and enjoy the good health that you deserve.