According to the Salesforce documentation, spam complaints refer to the number of prospects who marked an email as spam in an email report. An email report is a report that shows the performance and results of an email send, such as opens, clicks, bounces, unsubscribes, and spam complaints. A spam complaint is recorded when a prospect clicks the spam or junk button in their email client, indicating that they do not want to receive emails from the sender. Spam complaints can negatively affect the sender's reputation and deliverability, so it is important to monitor and minimize them. Spam complaints do not refer to the number of prospects who clicked the unsubscribe link, replied to the email to ask to be removed, or opted out via the email preference center, as these are different ways of unsubscribing from emails, not marking them as spam. Reference: Salesforce documentation