Interviewing, hiring, and onboarding is an expensive and time-consuming endeavor for any company. The interviewer wants to know that, if engaged, you could see this as a long-term fit rather than a temporary option. Also, keep in mind that Starbucks barista positions are quite competitive given the benefits that Starbucks offers to its employees. Thus, your interviewer wants to make sure that they are making a worthy investment by hiring you. Discuss your career plans, goals, and how those fit with what Starbucks is offering. Describe your passion for this opportunity and what you like about Starbucks, and the role for which you are applying.
If you are a student, you might tell the interviewer that you plan to stay at Starbucks through graduation and then share what career or further education you hope to pursue post-graduation. If Starbucks offers jobs in your area of study (for example, management or marketing), you can mention that you would be open to considering a career in your area at Starbucks if one is available upon graduating.