Afternoon is the time of day after midday or noon. Since people usually have lunch in the middle of the day, afternoon can be seen to be the time of day between lunch and dinner.
Breakfast is the first meal of the day. You can have all sorts of different things for breakfast. Many people have cereal with milk or toast with jam or honey. Some people prefer a cooked breakfast with bacon and eggs. Other people might like waffles or pancakes. And some people like a breakfast with almost everything, like in the picture.
Brunch is a late morning meal that serves a mix of breakfast and lunch dishes. The word brunch is a combination of the words breakfast and lunch.
Day might refer to the time when the sun is up, for example, "my cat sleeps all day", or it might mean an entire 24 hour period, usually counted starting from midnight. The opposite of day is night.
Dinner is the meal you eat at the end of the day, usually at around 5 or 6 o'clock. Dinner can be anything, but is usually some kind of meat and some vegetables.
Lunch is a meal you eat in the middle of the day. Most people have sandwiches for lunch, but you can have almost anything. Some people have salad for lunch, other people will have a steak.
Midnight is the middle of the night, 12am, or twelve o'clock at night. The opposite of midnight is midday, or noon.