Aggregator is a party which is involved in the secondary market. It purchases the mortgages from the financial institutions and then combines then into mortgage backed securities.
Aggregator earns profits by purchasing the individual mortgages at a lower price and then selling the combined mortgage based securities at a higher price.
A mortgage backed securities is generally less risky than the individual mortgages. The diverse set of mortgages provides a stream of income to the aggregator.
Many different types and processes for mortgage based securities exist but the most important point is that it pools the risk for the buyer.