A meal is everything you eat when you site down for breakfast, lunch or dinner. But, you can have different parts of that meal which are called courses. So, dinner is a meal and during dinner you might have an appetiser, a main dish and dessert. The appetiser, main and dessert are all courses.
I had a three course meal for dinner yesterday. The courses were a salad as an appetiser, fish as a main dish and ice cream as a dessert. It was a delicious meal.