People commonly use idioms or phrases that mean something different than the literal words. For example, saying "it's raining cats and dogs" doesn't mean animals are falling from the sky but rather that it is raining very heavily. Other idioms discussed include "it cost an arm and a leg" meaning something was very expensive, "hit the books" to study hard, "under the weather" to feel sick, "bun in the oven" being pregnant, and "tie the knot" to get married. The document emphasizes that idioms don't always make literal sense and examines some common examples.