A complaint is something the subscriber sends to mailRelate if he/she has marked your email as spam in his/her email application. If this occurs, the address will be unsubscribed in your address list and no further mailings will be sent to the recipient.
Garnering many complaints or bombarding an unsubscribed address can get you blacklisted and then you will not be ablet to send more emails.
You are able to view the email addresses that have complained in your rports, but never the reason for the complaint.