A coffee chat is an informal networking conversation with a banker. It usually gives you a chance to learn about the firm or group, build a professional relationship, and show that you could be a thoughtful candidate.
Most bankers will not turn the call into a technical interview. They are more likely to consider whether you communicate clearly, have done basic research, and show genuine interest. Expect questions such as “Why investment banking?” “Why our bank?” “Why are you interested in this group?” and “Walk me through your background.”