A hyphen is generally required when using more than one adjective, often referred to as compound adjectives or compound modifiers, jointly to describe a noun. Hyphenating is typically used when the compound adjectives are placed directly before the noun in the sentence, though there are exceptions to this rule.
A hyphen is always required between two adjectives describing a noun when, if absent, would change the meaning of the sentence or leave its meaning ambiguous. For example, the term small-state senator, with the hyphen placed between the words "small" and "state," clearly indicates the word small is describing the word state. Without the hyphen, the word small could be describing the senator.