The Yoast transition word assessment is part of the Yoast SEO readability analysis. It tells you whether you've used enough transition words in your text. You can find more information about it here: https://yoast.com/wordpress/plugins/seo/transition-words-check/

This assessment is language-dependent. To offer this assessment in a specific language, we need to know the transition words in this language.  If your language is not on the following list, we don't support it yet: https://kb.yoast.com/kb/features-per-language/

 You can help us to expand this assessment to your language by providing the necessary lists of transition words!