Accent marks are symbols added to letters to indicate a change in pronunciation, stress, or meaning. They are commonly used in various languages, such as Spanish, French, and Italian. For example, in Spanish, the accent mark (´) can change the meaning of words, like "sí" (yes) and "si" (if). These marks can also help distinguish between homographs, which are words that are spelled the same but have different meanings. In French, the accent grave (`) in "où" (where) differentiates it from "ou" (or). Understanding accent marks is essential for proper pronunciation and comprehension in languages that use them.