hyphen area？What does this mean? What is the use?

I like to describe it as how much "saw-toothing" you will allow. I like to set it to .25 inches on rag-right body copy.

https://helpx.adobe.com/indesign/using/text-composition.html

It sets the width allowable for empty line space if words are hyphenated on that and adjacent lines. Adjusting this prevents awkward breaks that leave some wide area at the end of a line blank.

Hyphenation Zone Specify the amount of white space allowed at the end of a line of unjustified text before hyphenation begins. This option applies only when you’re using the Single-line Composer with nonjustified text.

—https://helpx.adobe.com/indesign/using/text-composition.html

I can see you are using justified text text and InDesign defaults to the Paragraph Composer so please pay attention to the italicized text. 

~Barb