The word meal means any time when you eat a large amount of food. There are typically 3 meals per day - breakfast (in the morning), lunch (mid-day), and dinner (at night). A snack is a smaller amount of food (for example, a bag of chips or a piece of fruit) and you can eat a snack anytime.
In American English, dinner and supper are the same - both are used to describe the evening meal. However, in some cultures, the word supper is used for a light meal served in early evening, and dinner may be later in the night.