Q: We are supposed to eat every 2-3 hours in order to keep our metabolism fast and burn all the time.What happens in our body when we skip a meal and dont eat anything for 4-5 hours?

A: In case of just one skipped meal the changes will be pretty much negligible, of course. But in general keeping your body well-supplied with sources of energy (carbs, fats, and to some degree protein) and building-material for cells (protein, fats) allows it to be repairing and building the muscles as well as carry out all the other 'maintenance' processes that always go on in the background. When you don't supply your body with food for longer periods of times these processes slow down and your body starts to instead break down muscle for energy (in addition to breaking down fat for energy which is the preferred energy source for your body and for you, looks-wise)

It's always good to remember that an explanation like I just gave is a terrible over-simplification. What actually goes on in your body is a myriad of interconnected processes that the science is far from understanding, even now. But we DO know at a very high level how different foods and meal schedules affect muscle and fat development. It's a bit like knowing how to drive your car without understanding in any great detail how the engine and the transmission and the exhaust all work.