Understanding the Map Fields

Understanding the Map Fields

Can anyone explain the mapping of each Excel Column before importing my lead spreadsheet? 
After hours of search (terrible reference tables in this software site) I found the explanation for map fields. On the Page "Import Data to Zoho CRM" it details the excel column numbers, even though columns are letters, rows are numbers. However, it does not explain how I can change the map including the numbers. The example skips #2 & #3. Really confusing. One more day then I trash this and move onto another software. No way I want my employees spending days just trying to tweak their CRM software.