Answer: They help you find the page where a word is located.
Step-by-step explanation:
Guidewords in a reference book are usually located at the top of the page where they will be found. As reference books have some sort of order, seeing these words will let a person know whether they are close to find the words they are looking for.
For instance, in dictionaries, there are words on top of the pages. If you were looking for the word "cleanse" and you saw the word "clean" on top of a page, you would know that you are close to the word you are looking for.