If you are inserting, deleting, or updating the in the Salesforce files, then we use the Mapping Dialog window, to associate Salesforce fields with the columns of your CSV file:
If you want to match fields automatically with the columns, then click on Auto-Match Fields to Columns. At the bottom of the window, the Data Loader populates the list on the similarity of field and column names. Automatic matching only works on the ID field to delete operation.
Now, if you want to manually match fields with its columns, then click and drag the fields from the list of Salesforce fields which is at the top to the list of CSV column header names at the bottom, like if you are going to insert new Account records where the CSV file having the names of new accounts, and then click and drag the Name field to the right of the NAME column header field.
After the whole setup, find the Save Mapping button and click it to save this mapping for your future use, and also specify a name for the SDL mapping file.
If you have selected an existing file, then the contents of that file are replaced. And after click on Yes to confirm this action, or click No to choose another file.
At last click OK to use your mapping for the current operation.