This topic is very wide open. You could choose to write about an extracurricular activity, job, hobby, or just about anything that involves a high level of interest and dedication. Most of our suggestions forAccomplishmentsessays also apply here, especially refraining from repeating information found elsewhere in your application and not including a laundry list of academic, extracurricular, and work successes.
Focus on why the interest is important you, making sure to highlight its relevance to other areas of your life and what you have learned from it. Demonstrate passion, devotion, and leadership skills, as these are all character traits admissions officers seek in future college students. Most of all, be genuine—admissions officers will know if you are telling them what you think they want to hear.