"CSV" is a file format which is popular because it is relatively simple and "human-readable" so it is universally understood and widely supported. CSV stands for "comma separated values" and it is, literally, a plain text file containing data which is delimited by commas.
Every line represents a separate entry - for example a person in your address book or a single transaction on your bank account - and the separate data values for each line/entry are separated by a comma.
Eg:
FIRST NAME, LAST NAME, FAVOURITE COLOUR
Joe, Bloggs, Purple
eLEDGER allows you to use CSV files to get both your banking data and contacts into the system, since this file format is supported by all Internet Banking and all email/addressbook platforms.
Wikipedia has more in-depth information on the format.