what is title-tag?

I noticed in the new version of WordPress it says we can now add support for title-tag. What is title-tag? Is this something I would find useful? Should I add that line to the function.php of my child theme?

I have read the Codex and also the official announcement and none of it makes any sense to me.

Some of my posts use italics in the title, which looks just fine displayed on the page, but if shared with Facebook for instance, it tends to show the html tags, which looks ugly! If this new title-tag can prevent that, I want to know how to use it! 🙂

Otherwise, please tell me what it is meant for.

Thanks!