Here you select which of the message fields the program should look for the filter string. The message filter allows you to tell the program which messages should be parsed with this template. For example you might only want to include messages where the subject line contains "Online Survey Results". In this case you would select "MSG_SUBJECT" from the drop down list.
Select <None> from the drop down list if you want all messages from the mail source to be included in the email processing.
Here you enter the string that the filter is to look for. Any messages that contain the defined string in the selected message field will be included in the parsing results. All messages that do not contain the string are ignored.