At first, you should take an abuse alert as a "heads up" that people are complaining about your campaigns. If you get more complaints than a small handful, you should be concerned and we highly recommended to re-evaluate your email marketing process to prevent further complaints.
I got an abuse alert in my account
Once abuse complaints exceed our threshold the account will be suspended pending a human review process. They will ask about your list collection process and ask for other details about the health and life of your list. We'll need that information because by then ISPs and anti-spam organizations are threatening to blacklist us unless we explain why your list generated so many complaints. A full investigation will proceed once we have the requested information from you.
Even if you're a legitimate marketer who does everything "by the book" and only uses opt-in lists, you can still get reported for spamming. Basically, when a recipient gets your email and thinks it's spam, or just "junk mail," they can click the little "Report Spam" or "This is spam" button in their email application (even if they signed up for it!). Some people just think the button is an easy way to unsubscribe from your list. Some people are just too lazy to scroll down and click your opt-out link. That's why a couple of complaints are understood, but several gets into sticky territory.