If you eat more calories than you burn, you will put on weight.
If you burn more calories than you eat, you will lose weight.
why countries like Thailand aren't obese
If you are living in a country like Thailand, it's unlikely you will have the opportunity to eat more calories than you burn. Until very recently, for example, in India being overweight was a sign of high status (still is to some extent).
If you are living in the UK, you have much more opportunity to over eat, hence many of us are keen to find a way of eating that makes us feel full and prevents us eating more. That is different for each of us.
Calories come from three sources - carbs, fat or protein. If you drop one type, you have to eat more of the others. Low carb = more calories from fat or protein; high carb = fewer calories from fat or protein.
if you are trying to avoid junk food and fatty foods
Body fat is not the same as dietary fat. There are bad and not-bad dietary fats, e.g. you cannot compare an avocado with a Mars bar in terms of what you get from each, but both are high fat food items.
There is a great deal of misunderstanding about food and diet, propagated by (a) our human desire to over-simplify and find a magic bullet to all our problems, and (b) the media which relies on over-simplification of complex issues.
Carbs do not make me slim, they make me heavy, bloated, tired and craving more. But I luffs them anyway. That is my problem, some people will have a similar problem, others won't and will do better on a low-fat, higher-carb diet. None of us is wrong, but it's a lot of work sometimes to find what's right for you.
Here endeth the lecture... 