In case that info confuses you even more... the title of the window in the screenshot isn't really important and is probably best ignored.
It's just a normal Lister window, the main window of Opus that shows files and folders.
If you click where the files & folders are displayed, then type a > on the keyboard, a green "Command" box will appear where you can the command in. The command, from earlier, is:
Set AUTOSIZECOLUMNS=widest
So just open an Opus window, type > and then paste that into the green box that appears, and push return.
The green box lets you run the same commands that you can put in toolbar buttons and hotkeys. If you want to use the command often, you'll probably want to create a button for it rather than type it in every time.