Readability Tab

The Yoast SEO website states:

The readability analysis uses an algorithm to determine how readable your post is. We’ve carefully crafted this algorithm to make it as accurate as possible without being too strict. It features several checks that’ll give you advice when you write your post. In other words, by following the advice, you can make your text easier to read and understand.

Visit the page for more details.

Yoast SEO readability