Any job that requires customer relations means that inevitably you will find a difficult client. Employers know this, and hires do too, but not everyone handles this sort of client well. Sometimes in the face of conflict, workers escalate the situation rather than deescalate. This can result in arguments or physical altercations. Either way, this reflects poorly on a company. When this question is asked during an interview, the employer wants to know how you handle such situations. Providing an example in addition to an answer is best practice.

"What has helped me work with difficult clients is recognizing their humanity. I'm not perfect. I make mistakes and bad decisions. Like me, maybe they are having a bad day, or maybe no one has addressed their behavior. If someone is being difficult, I tolerate what I can, reminding myself of this. I also recognize that I'm working with them at the moment, but not forever. If the conflict becomes too much, then I calmly talk to them and figure out how we can move forward."