https://store.office.com/message-header-analyzer-WA104005406.aspx?assetid=WA104005406
What it does is allow you to inspect message headers right from the Preview pane, or if you don;t use that from the email itself without having to pull up the Properties and then copy/paste from that tiny little window into Notepad or something in order to read it all.
The bonus though about this add-on is that it breaks the headers down just like Microsoft's Header Analyzer from: https://testconnectivity.microsoft.com/MHA/Pages/mha.aspx
Here's what it looks like from within Outlook
