Asking you this question is the interviewer’s attempt to determine your key interests and whether the job on offer has tasks or responsibilities that you will like or dislike. For the positive aspects of your last position, things are pretty straightforward.
First, focus on the parts that you enjoyed the most, explain what you learned from them, and then talk about how they made you develop as an individual.
On the flip side, you left your last job for a reason — sometimes a variety of reasons — but the key is not to take this opportunity to air your grievances. Instead, be mindful of criticising your employer. Unless your former boss was truly a toxic individual, complaining about him or her can come across sometimes as you pushing the blame on others.