Hyphens are used in many compound words to make the meaning clear. For example, ‘man-eating tiger’ tells you that the tiger eats people – ‘man eating tiger’ could mean that a man is eating the tiger.
Sometimes hyphens are used to join a prefix onto a word. Prefixes are morphemes (groups of letters that mean something on their own) that are added at the start of a root word to change the meaning. While most prefixes are joined onto words without a hyphen, they can be useful to avoid confusion with another word. For example, ‘re-cover’ (giving something such as a book a new cover) has a different meaning to ‘recover’ (get well after an illness).