In the simplest terms, a blunder is a big mistake in backgammon. When one plays backgammon, he can make two types of mistakes: making a very bad cube decision or moving his checkers in some other way than the most favorable. All these types of mistakes are considered blunders, and if a player makes such mistakes, his opponent will have a better chance to win. If a player wants to be aware of the most types of blunders, there are articles and even books on the topic. The most important thing is to avoid blunders, and if the happen, to learn from them!