I've not researched the Stan essays yet .. so I am sure kryzak will have better inputs to give. But let me try analyzing it.
They are giving you a lot of freedom in defining "impact" (By hinting that the impact can be on a person or group). I think "impact" simply means anything that changes / effects a person or group. If you talk about impacting a person, the impact can be at a very personal level (But not necessarily). If you talk about impacting a group, the impact can be at a professional level (Again, not necessarily).
Now, which impact you find "significant" is part of what they are trying to get from this essay. So, which impact is "significant"? The one that you think is "significant". Which impact did you value more in terms of significance will tell them a lot about who you are.
Finally, what does "a time" mean? Again, it can be a single event or a time-period. Nothing very tricky about it. Basically "a time" just hints that perhaps you should stay away from life-long impacts. So you can't write an essay about how you've significantly impacted your younger brother over all your life.