Dealbreakers are things that you are NOT looking for on RedHotPie. In the same way you can add things that you want to find on RHP (eg. Relationship, Online Fun etc.), you can also specify things that you don't want such as Sugar Dating and Open Relationships.
Once you add Dealbreakers, you'll see any conflicting interests on someone's profile highlighted in red. You can then assess if that person is likely to be a match before contacting them. You'll also be warned when you start a message to someone if they have a Dealbreaker that you're not aware of.