An abuse complaint happens when you send a communication and the recipient clicks on the "mark as spam" button when they receive the email.
When this happens several times you are flagged as a spammer and then all the emails you send in the future are marked as spam and delivered to the Junk/Spam/Bulk folder, thereby lowering your response rates.
There may be many reasons why your recipients are marking your messages as spam, but the only way for you to avoid it is to follow email deliverability best practices.