CSV stands for "comma-separated values", a specific format of spreadsheet file you can export from most spreadsheet-editing software (like Microsoft Excel, Apple Numbers, or Google Sheets).
To export a CSV spreadsheet from Microsoft Excel:
Open the file you want to save.
Navigate to File > Save As.
Select the arrow in the Save As drop-down box and pick CSV.
Hit Save to complete the process and produce a .CSV version of your file.
To export a CSV spreadsheet from Apple Numbers:
Open the file you want to save.
Navigate to File > Export To > CSV.
On the Export Your Spreadsheet pop-up box, choose Next.
Hit Save to complete the process and produce a .CSV version of your file.
To export a CSV spreadsheet from Google Sheets:
Open the file you want to save.
Navigate to File > Download > Comma-separated values (.csv).
The .CSV version of your file will be downloaded by your browser immediately.