In the United States, breakfast is eaten from perhaps 6 a.m. to 9 a.m. and is often a fairly light meal with a fairly limited and stereotyped number of food possibilities. A cup of coffee, a roll and butter, a bowl of cereal... perhaps an egg, perhaps scrambled eggs and bacon. Lunch is eaten at about noon, and can be light, medium-sized, or heavy.
Brunch is intended to combine breakfast and lunch. If you are eating brunch, you would not eat breakfast first, and you would not eat lunch afterwards. So, it is a large meal. And it is eaten in the middle of the morning, perhaps 10 or 11 a.m. It would contain a good selection of breakfast-like and lunch-like foods.