In this video, the teacher explains the concept of idioms and how to understand their meanings through context clues. Using the example of "water under the bridge," the teacher encourages students to think beyond the literal interpretation and consider the metaphorical meaning. By visualizing the flowing water under a bridge and understanding its transient nature, students can grasp that the idiom refers to past problems that no longer affect the present. The video provides a step-by-step approach to deciphering idioms and emphasizes the importance of context in understanding language.