Long-vowel symbols, also known as macrons, are diacritical marks used to indicate a long vowel sound in a word. They are represented by a horizontal line above the vowel and are commonly used in languages such as Latin, Greek, and Hawaiian. Macrons are used to differentiate between words that have the same spelling but different pronunciations. For example, the word 'mae' can be pronounced either as a short vowel or a long vowel depending on whether it is marked with a macron or not.