Im assuming you are refering to the aggregation setting found on various measures across the app mon environment.
In short Aggregation is the type of data you would like to see presented. Its best to think of it in terms of a chart dashlet.
Lets say you chart overall web request response time on a chart dashlet. Choosing the aggregation dictates what the points on that said chart represent. So for example, if you chose average as the aggregation, your plotted points on the chart would represent the average response time for that given time frame and resolution (so a 5 min resolution would result in one point showing the average for that given timeframe). Hope this helps.