When you hyphenate a word, you use a punctuation mark that resembles a dash to connect two words into one or separate a word's syllables. To write the word old-fashioned, you have to hyphenate it.
When you write words like deep-fried and record-breaking, you hyphenate them, or add a hyphen to join their separate parts into one word. Another common reason to hyphenate words is when you reach the end of a line and need to split the word you're writing into two parts, continuing the second part on the next line. The rule here is to hyphenate between complete syllables.