Beside every completed import, you'll find an options button to "View import statistics." If you choose that option, you'll see a list of numbers explaining your import.
Below is a breakdown of each total that appears and what it means.
This is the date and time at which your import finished.
This is the file name from which you imported.
This tells you whether your import style was "add only" or "add & update."
This is the list of member fields -- email, first name, last name, etc -- into which you mapped.
This is the group to which these members were imported.
This is the number of email addresses that appeared multiple times in the file.
This is the number of blank rows in the file where no email address was populated.