OK, Debbie, first and foremost, make sure you have a backup of your "stuff".
Secondly, there are two things, at least, that you'll need to know initially. First of all, the "location" of all your rtfd files. If they are all in the same folder, that will make it easier. The easiest way to find that out is by entering the name of one of the rtfd file names in the Search box for whatever Search tool you use. Most folks use Spotlight (comes with Apple's OS), which is fine. (I use a free program called "EasyFind" for my searches). Once the file is found, the entire pathname should be shown, and that is what you would enter in the terminal string "/Users/username/path/folder-name".
The "string" "*.rtfd" at the end of that command just means that the conversion to docx will occur only for files in that folder whose name ends with .rtfd.
Next, the Terminal "program" is location inside the Utilities folder, which is inside your Applications folder. To get to it, double click the Applications Folder, then double click the Utilities folder, then double click on the Terminal program.
If your rtfd files are "scattered" in different folders, then you'll need to repeat that Terminal command, except that the "folder-name" would be different each time.