In the morning, you have breakfast and in the middle of the day, you have lunch. In the evening, you might have dinner or supper. The difference between these two is mainly their size.
Dinner is a main meal, ‘more complete’ than supper. It usually consists of a first and a second course, and possibly some dessert. So people who have something light for lunch, for example just a sandwich, usually have dinner in the evening. It is their main meal.
Supper, on the other hand, is not the main meal. It’s just something light before going to bed. It may be a sandwich, a burger, some salad or soup.
In Britain, the main meal is usually dinner (in the late afternoon or early evening). Lunch and supper are lighter meals.