Diacritics are special symbols added to letters in various languages to modify their pronunciation or meaning. For example, in the word café, the acute accent over the 'e' indicates a specific vowel sound, distinguishing it from the plain 'e'. These marks are essential in languages like French and Spanish, where they can change the meaning of words entirely. In addition to pronunciation, diacritics can also indicate stress or tone, as seen in languages such as Mandarin Chinese. Understanding diacritics is crucial for accurate reading and writing, as they provide important phonetic and semantic information that can alter the interpretation of text.