Aggregator

There are, of course, many kinds of aggregators. In media, an aggregator brings together content, i.e. you buy an ESPN bundle through Comcast from ABC-Disney. In the financial world, an aggregator brings together mortgages. Way less entertaining.

The financial type of aggregator buys individual mortgages from other financial institutions, the kind of mortgages regular people get for their homes. Then, the aggregator packages the mortgages together into securities, which can be traded on the open market.

These mortgage-backed securities can be bought and sold like bonds, making the collection of mortgages more liquid (i.e. more easily bought and sold) than each individual mortgage would have been.

Think of it like buying a bunch of individual car parts. A couple fenders, a couple bumpers, a hood, some windows, all the separate engine parts, etc., etc. Then put these together to build a working car.

The car you've built is easier to sell than the parts would have been individually. The finished car has more general use, therefore it's easier to find a buyer. The parts were each very specific, so they only appealed to certain people. The completed car has appeal to a bigger market.