A complaint is when a contact clicks the "spam" button in their inbox. Email Marketing tracks these through feedback loops and reports them in your statistics under the "Marked As Spam" stat.
The best way to reduce spam complaints is to alter your opt in process. Think about the reasons why a contact in your list would mark your email as spam. Whatever the reason, it's always about an unwanted email. When a contact reports your email as spam, they're saying they did not expect to receive an email from you and do not want to receive another! So go through your opt-in process as a test and ask yourself if you can make any step clearer.