The document defines compound adjectives as two or more words that act as a single idea to modify a noun. It provides examples of compound adjectives like "part-time employee" and "high-speed chase". It explains that a hyphen is used between the words to indicate they act as a single adjective. The document then discusses different types of compound adjectives formed from combinations of numbers and time periods, adverbs, nouns, adjectives, and participles. It provides rules for when a hyphen is needed with each type of combination to create a compound adjective.