I will meet you in the mall.
and
I will meet you at the mall.
The first sentence implies that you are going to meet the person inside the building. Where exactly inside the mall you are going to meet the individual, we don’t really know. All that we know is that the meeting is going to take place inside. When someone informs you that he will meet at the mall, you are not really sure whether it is inside or outside the building. It could be either.
