It is important that the data you would like to upload to Explorer is on the first sheet in your Excel file and that it contains a header row, with informative titles such as "text", "customer reference" etc. For Excel files containing multiple sheets, only the first sheet is processed by the Explorer, so be sure to arrange or split the file accordingly.
You can use a tool like http://csvlint.io/ to check if your file is a proper CSV. You can Press Save as in Excel to choose CSV format. Another option is to upload the file to Google drive, open it as a Google spreadsheet and then download it as a CSV (File → Download as → CSV). A third option is to open your original file in LibreOffice and save as CSV (this has the added benefit of enabling you to change the encoding if you want). All of these options give you good CSV files that adhere to the standard about which you can read more here: https://en.wikipedia.org/wiki/Comma-separated_values.
If you export from Excel you should note that you need to make the file comma-separated and not semicolon-separated.
For more information on file formats see section 2.1.1 of the documentation.