The document describes how the author lost a chess match in a school competition after underestimating his opponent. This loss caused him to realize he had a conceited attitude where he did not take opponents seriously. He then made an effort to change his attitude and treat all opponents with respect. He communicated this lesson to different people, including his parents and the opponent he lost to, and found they were supportive and wanted him to learn from the experience. The author now warns others against underestimating opponents based on his own lesson.