The focus keyword is a field where you can insert a keyword or phrase. Once the keyword or phrase has been added, the plugin evaluates the page or post—including the SEO title, H1 tag, body copy, and other elements—to let you know how well you've optimized for your keyword or phrase

Focus keyword is a field you have to decide where you can insert the keyword. It is the keyword you want page to rank for.

Its a field where we inserts a keywords or phase. the plugin evaluates the entered keywords or phrase. Which helps us to know that is our keyword or phrase is optimized.