A "deal breaker" is a quality or point that you can't accept. Whenever you're considering a decision, there are some things you don't like but can live with. But a "deal breaker" is something that you can't live with. You use this phrase to talk about negotiations in business situations:
We liked their work, but the rates they were asking for were a deal-breaker for us.
This sentence means that you definitely won't use this company's services, because they're too expensive.
Or you can use "a deal-breaker" to talk about relationships like in this example:
Smoking is definitely a deal-breaker.
In that case, it means that you definitely won't date someone who's a smoker.