Your interviewer is interested in assessing your skill level and how organized and effective you are. Multitasking does not come naturally and can be a more difficult skill to master for some than others. So, your interviewer asks how effectively you organize, track, and manage your work when juggling multiple priorities and whether you can effectively address them without making any mistakes. As they detail on many of their job postings, they are looking for candidates who have an "ability to use time efficiently and productively. Ability to demonstrate assertiveness and influence, when appropriate." Assure the interviewer that you can be diligent when assessing your priorities and have systems and processes to ensure you follow through on all the tasks you have taken responsibility for.