Aggregation in Endeca allow developers to do two things:
1) During navigation the results can be "rolled up" (synonym for Record Aggregation) based on a shared property or dimension value.
2) When the user requests the details page by clicking the link for a specific item, an Aggregate Record query is performed to return records of that item type.
You have a list of results. Based on shared property or dimension values, records in this list can be "rolled up". Otherwise you will have a list 12 results instead of 4 in the example below.
Product Type = Sweater
Results: 1) A Sweater (3 Products)
2) B Sweater (5 Products)
3) C Sweater (2 Products)
4) D Sweater (2 Products)
In a record details page, all aggregated records can be requested.
Example: Product Details
1) A Sweater
2) A Sweater
3) A Sweater