Microsoft Excel can read .csv files, but so can many other programs. When Excel is installed on your computer, it automatically associates the .csv extension with Excel. This is why the files show with an Excel icon.
If you look closely at the icon, you'll notice it's slightly different than the icon that's normally used for Excel spreadsheets / workbooks.
Transfer to your GPS
To transfer the file to your GPS, you'll need to use software like Garmin's POI Loader tool. (You don't double click on the file. Instead, you need to start POI Loader and tell it where the file is located.)
Opening with Wordpad or Notepad
You can also open and edit .csv files with Wordpad or Notepad. On a PC, just right click the icon and select "open with".