It's a horizontal line placed over the vowel letter. For example:
* ā represents the long "a" sound, as in "fate"
* ē represents the long "e" sound, as in "tree"
* ī represents the long "i" sound, as in "time"
* ō represents the long "o" sound, as in "go"
* ū represents the long "u" sound, as in "flute"
Sometimes, you'll see other markings used to represent long vowel sounds, such as a colon (:) or a breve (~), but the macron is the most common symbol used in dictionaries and phonetic transcriptions.