When you're writing, a hyphen is useful for forming a compound word or separating the syllables of a word at the end of a line of text. The word merry-go-round uses two hyphens.
Often, when you join shorter individual words into one more complicated, compound word, you use a hyphen. Words like twentieth-century, well-intentioned, and high-risk are examples of hyphenated compounds. Hyphen is a Greek word that means "mark joining two syllables or words," which most likely was originally used to describe the way words should be sung.