You yell at them (MS) because things are too hard, some of us yell at them because the keep trying to make things simpler for you, making things harder for us.
"Compatibility mode" is one of those "make it simpler" things they did. I believe there was more reason when they used the old "DOC" format files, but for some reason they continued it in the new DOCX format. So when you open an older file it shows the
"Compatibility Mode" information message in the title bar as a clue when you try to use a command and it does not work.
Here is a MS page that lists the features that trigger this mode in 2007 and 2010:
https://support.office.com/en-us/article/Compatibility-changes-between-versions-cb713c85-3145-4e83-a8b6-1e3227a4c059?CorrelationId=081cb0a8-4b6d-42da-b1b5-94ed142ff4d0&ui=en-US&rs=en-US&ad=US&ocmsassetID=HA010370116
What compatibility mode means is that the file was created in an older version of Word, and it cannot support all of the features in the newer version of Word you are using. The one I specifically remember was when "Text Effects" (the fuzzy blue "A" icon
in the Home tab > Font group) in 2010. When you opened a 2007 DOCX or 2003 DOC in 2010 (or later) the blue "A" is disabled, so it is displayed "greyed out".
Here are 1-2-3 (litteraly) how to instructions.
https://www.techwalla.com/articles/best-wireless-gaming-headsets-of-2016
let us know if you want anything explained
This page has more information about the subject in a format you hopefully can understand
**What Is “Compatibility Mode” in Microsoft Office? https://www.howtogeek.com/256269/what-is-compatibility-mode-in-microsoft-office/**by Chris Hoffman on May 28th, 2016
When you open a document created in an older version of Microsoft Word, Excel, or PowerPoint in a modern version of Office, you may see “Compatibility Mode” appear after the name of the document in the titlebar. This changes the way the document
appears and prevents you from using some modern features. This normally isn’t something you should worry about–Office does the right thing to ensure old documents work properly and you can continue to collaborate with people using older versions of Office.
But you can leave Compatibility Mode, if you like.
Hint: I just posted a suggestion to MS to automate conversion to make it even simpler. Please go to the link and vote for it and add your own comments to it. Maybe MS will listen and do it ...
Compatibility mode - automate conversion - https://word.uservoice.com/forums/304924-word-for-windows-desktop-application?query=Compatibility%20mode%20-%20automate%20conversion
If you want more general and technical information about compatibility mode you can look at this page:
Plan for using compatibility mode in Office 2010 https://technet.microsoft.com/en-us/library/cc178998.aspx?f=255&MSPPError=-2147217396
Although standardizing on the Microsoft Open XML file format is the best way to minimize compatibility issues, this goal can be difficult to achieve for organizations that plan to deploy Microsoft Office 2010 over a period of months
or years. Even after the migration is complete, users might continue to collaborate with partners, customers, and other organizations that use earlier versions of Office. To help users maintain productivity during all phases of an Office 2010 migration, you
can let users continue to work in the 97-2003 binary file format (*.doc, *.xls, and *.ppt) and use the compatibility features that are included with Microsoft Excel 2010, Microsoft PowerPoint 2010, and Microsoft Word 2010.