My granddaughter's first-grade reading papers frequently use the term "high frequency words." I'm guessing it refers to words used frequently. But, if the term "high frequency words" is correctly used, shouldn't it then be "high-frequency words"?
The short answer: it doesn't require a hyphen.
The longer answer:
Generally speaking, you don't need a hyphen in a compound modifier (like "high frequency") if there is no ambiguity without one, according to the Chicago Manual of Style.
If "frequency words" could be considered a compound noun, then we would need a hyphen there; since it cannot, we don't.